Recommend me a bike...

[whoof]

If you require a L sized bike and are 72kg then it doesn't sound like you are carrying way too much weight. The tyres are reasonable mid-range but then you need some reliability.
With gearing no one can tell you that a certain gearing is right or too high, everyone is different.
28" is slightly higher than I used (26" ) but I was carrying luggage. It is the same as I will be using in a little over a week in the mountains of Majorca.
You have time try some back to back long rides with long climbs and see how you feel. If you find the gearing too high you may be able to fit a cassette with a 40t sprocket and possibly a smaller (36t?) Chainring.
As recommended above increase your fitness and I'm sure you'll have a great time.
Edit. One other thing I meant to say was are you comfortable on your bike as you say it's not the correct size?

[Scunnered]

Ha ha, yes I'm as lean as a racing snake.
I know I will need a lower gear. I can fit a 42t sprocket if I get a long cage derailleur and may be able to fit a 36t chainring.
The furthest I have ridden is 100 miles. I was pretty tired at the end so wasn't thinking about comfort. Not knowing anything else, I suppose I have got used to the bike. I did fit a shorter stem when I first got it.

[slowster]

Things that I think you need to consider:

1. Firstly, you state that your current bike is suitable for most of your riding. Will long road rides/tours, potentially involving multiple days in the saddle be something that you are going to do a lot more of, or will they only ever be a small part of your riding. Do you envisage doing less riding for which the gravel bike is suitable, and more on road? Or are you going to do as much of that type of riding as before, and you will also be doing a lot more road riding as well?

2. You're thinking about gettting another bike, which will be more suitable for road riding and the sort of riding that the event will entail, and that bike will complement your gravel bike. However, there is a lot of overlap in your likely needs for both bikes and type of riding. For example, you could get an out and out road racing bike, which would be very light, but would be too highly geared for anyone not a very fit road racer, would be unsuitable for anything other than the smallest seat pack, and would not have the option to fit proper mudguards (you presumably would be committed to riding even if it rained all day, possibly for multiple days if you were unlucky with the weather).

3. I don't think 38 x 36 is low enough, especially for multiple consecutive days with long climbs. If cumulative fatigue starts to set in or you have a bad day (something which can happen to us all), gears that might have been fine for the first climb on the first day when you were fresh will soon seem far too high.

4. You can save a lot of energy if you ride in a group. Learning to ride relatively close behind another rider and take advantage of their slipstream is likely to be a very valuable skill for the sort of event that you will be doing. A few riders who can ride reasonably close together will be faster and get to the destination in much better shape than individuals who ride pretty much solo, especially if there's any headwind. Riding in a group also helps morale.

However, in a group you do not control the pace, and you need to be able to match the pace of the other riders. In that regard your current 1 x 10 range of gears will probably have gaps that are too big for you to be able to choose a gear that does not feel like your cadence is too high or too low when trying to match the speed of other riders. (If you are not familiar with group riding, consider joining a local club and going on their weekly club run and/or enter some local short distance audaxes or club reliability rides, e.g. 100km or 50 miles, and use them as an opportunity to develop group riding skills).

There are plenty of bikes that would probably fulfil the above criteria, but given the overlap between the sort of riding that are already doing and the type of riding that you are planning to do more of, I would think seriously about getting one bike for both types of riding. The fact that you think your current bike is not the right size means it makes even more sense to get just one bike, make sure it fits properly, and get the best bike you can afford rather than have two bikes which are a compromise with lower specifications (and one of which may not fit properly).

There are a number of frames (as opposed mostly to complete bikes) that would be capable of being built up to fit your needs. You mention Kinesis, and the aluminium Kinesis AT is one such frame. To give you a very general idea of the sort of build/specification I would be considering, the Spa Elan with 105 triple would probably be a good starting point, but I would change the standard spec for lower gears, probably replacing the 50/39/30 chainset with something like 48/36/26, and maybe choosing an 11-34 cassette instead of 11-32. With a well chosen triple set up and wheels that could take both a fast rolling 28mm tyre as well as much wider tyres for off road use, and given the ease with which mudguards could be removed and re-fitted on a disc braked bike to suit the weather, you should have all bases covered in one bike.

[Scunnered]

Things that I think you need to consider:

Thanks for your thoughtful post.
1) I do not at this stage anticipate doing long tours very often (I prefer hiking when I have the time or choice) and will continue to use the gravel bike off-road.
2) A degree of functional overlap is acceptable and poss even desireable.
3) Agree that my current gearing is not low enough. Thinking about 1x11 11-42T or MTB double. I use SRAM so some road and MTB components are intercompatible.
4) New Spa Elan is well above my budget, will look for something 2nd hand or ex-display.
Cheers

[PH]

First - experiment with the sizing of your current bike. You think it may be the wrong size, but the way to test that isn't by buying another bike that may also be the wrong size. Position is the important bit, most people can replicate their preferred position across a range of sizes.
Then, decide if you're looking for a replacement or an additional bike. One of the nice things about having more than one is they don't all have to be practical.
You still haven't said when the trip is. I wouldn't worry too much about gearing yet, what you'd need to do it now or in X months time could be different.
As others have said, your current bike would do fine, on a budget the gains are going to be minimal, but if we all applied that criteria there'd be no bike industry left. Just don't think that you have to have a new bike, or that there's any rush to get one, certainly don't put off your preparation till you have it. On the ride I guarantee if there's big differences between riders, the bikes will be a very minor part of it.

[slowster]

Spa Elan is well above my budget, will look for something 2nd hand or ex-display.

I was not intending to suggest that you get a Spa Elan, but rather that that particular build would be closer to the sort of specification I would look for on that general type of bike for all the types of riding that you will do. As I said, frames like that, such as the Tripster AT, tend either to be built up with 1 x set ups etc., which are arguably less suited for your needs, or are sold as framsesets only.

You could just get another bike, but that would still leave you with a gravel bike which you say probably is the wrong size.

Another frame in the same category as the Tripster AT and the Elan, but a lot less money than both, is the Sonder Al, which retails at £300. If your current bike is indeed the wrong size, then you could buy a Sonder and transfer the parts from your current bike, to give you a gravel bike that fits you properly and which would also be quite a bit lighter by virtue of the aluminium frame and carbon fork. You could then use the bike as a test bed to tweak and experiment with the set up, especially the gears, to see what would work best for you on long road rides with long climbs.

[PH]

quite a bit lighter by virtue of the aluminium frame and carbon fork.

I can't see where Scunnered has said what their current bike is, though as an "ubiquitous gravel bike" weighing 11 kg, I'd already assumed it would be an aluminium frame and most likely a carbon fork. Any weight saving is likely to be counted in the 10's of grams when compared to another gravel bike (Of the same size) if all they do is transfer the same components. Changing for a rim braked winter trainer might get that up into the 100's of grams. Either way it's going to be of little significance, certainly less than say an extra 30 min serious training a week between now and the trip.

[Scunnered]

quite a bit lighter by virtue of the aluminium frame and carbon fork.

I can't see where Scunnered has said what their current bike is, though as an "ubiquitous gravel bike" weighing 11 kg, I'd already assumed it would be an aluminium frame and most likely a carbon fork. Any weight saving is likely to be counted in the 10's of grams when compared to another gravel bike (Of the same size) if all they do is transfer the same components. Changing for a rim braked winter trainer might get that up into the 100's of grams. Either way it's going to be of little significance, certainly less than say an extra 30 min serious training a week between now and the trip.

It's a Pinnacle Arkose, which is Al frame and carbon forks. With all those oversize tubes it looks "robust" rather than "svelt"

[slowster]

I can't see where Scunnered has said what their current bike is, though as an "ubiquitous gravel bike" weighing 11 kg, I'd already assumed it would be an aluminium frame and most likely a carbon fork.

My mistake - for some unfathomable reason I thought the OP's current bike was steel.

[The utility cyclist]

If you're getting another bike, get one with a double set up not a 1x and similarly if you're upgrading the current bike get a double set up, a sub compact or a 50/33, with the 36T sprocket that's ample lw gear. As per NUKE, uprgading to best tyres and tubes you can afford plus some decent wheels plus a nice shiny chain and good service.
You can buy some really nice wheels for not very much these days. Whilst you thinking fitting a 40T cassette sprocket might work in terms of getting the low gear you're increasing the jumps between the ratios, under duress this is detrimental whether racing, charity ride, touring or any other riding.

[Scunnered]

Replaced the single crank with a double 38/28T.
With a 11-36 cassette, that should be low enough to get up any hill... just need to work on my fitness now

[keyboardmonkey]

Replaced the single crank with a double 38/28T.
With a 11-36 cassette, that should be low enough to get up any hill... just need to work on my fitness now

And does everything work okay with that front mech and chainset combination?

[Scunnered]

Replaced the single crank with a double 38/28T.
With a 11-36 cassette, that should be low enough to get up any hill... just need to work on my fitness now

And does everything work okay with that front mech and chainset combination?

Yep, with SRAM road brifters

[keyboardmonkey]

Replaced the single crank with a double 38/28T.
With a 11-36 cassette, that should be low enough to get up any hill... just need to work on my fitness now

And does everything work okay with that front mech and chainset combination?

Yep, with SRAM road brifters

Result!