Thanks for all the speedy answers!
If you have fitted the chainring to the outside of the spider, then your chainline should be a lot more than 49.2mm
I measured the chainring line at 50mm using a ruler, so assumed that was pretty much what Shimano said should be 49.2mm.
I suggest you also check that the 6.5mm spacer (item 11 in the exploded diagram above) is fitted
The crankset doesn't have any guards on it and the spacer only applies to the double guard. There's no spacer, and I think my crankset must be older than the one you linked to.
Look at the chain on the ring and sprocket, it shouldn't have the side plates in contact.
The teeth on the chain ring and sprocket seem to be quite happy, not nudging the sides, although they do wander a little from front to back, showing the chain is at an angle. So perhaps not desperately urgent to sort out, but I know it's there, it'll bother me until I do find a solution (only noticed because I needed a new sprocket on the Rohloff - presumably I've been running an angled chain for the last 30,000 miles!)
How are you tensioning the chain?
Horizontal sliding dropouts (if that's what they're called), so there's nothing other than the chain ring and the sprocket in contact with the chain.
Get a long straight edge rather than trying to calculate front and back chainline, and see how bad it is that way.
That was a brain teaser - no straight edges to hold the straight edge against and compare the chain line to! I put a spirit measure against a couple of chain bolts (then another two to average it out) to see where the other end of the measure ended up. It looks like it's approx 5mm (spirit measure was too wide and the spokes were in the way, so had to extrapolate the last couple of inches).
All in all, it sounds like 5mm is worth worrying about, and the easiest solution seems to be spacers with extended chain bolts. My current bolts are m8/8.5 and are 1mm too long, the chainring is 2mm thick and if I use 5mm spacers then presumably I need to find some bolts and sleeves that are about 12 to 14.5mm long? Am I seeing that right, that the spacers slip over the bolts and sleeves?
Thanks for your help so far - it's got me a lot further!