Yes, resolved, but quite a palaver!
I contacted St John Street Cycles, and got a helpful reply from Josh, who pointed me to a Shimano hp-m-563 crown race which is 27.0mm dia. He also noted "This fits with the majority of the 1” sealed bearing headsets with a slight modification. All you need to do to get this to work perfectly with the A9 headset is file back its outer lip ever so slightly ". This was good advice and I was happy to order one (which cost me £7.60 inc postage) though I had misgvings about trying to file the cup of the A9 - it's hardened! If it wasn't hardened, I could simply have stuck the original A9 crown race in the lathe and turned it up to 27.0mm but the cone needs to be hardened if it is to last any length of time. As predicted, the Shimano crown race was slightly too large in diameter to fit (just) inside the A9 cup, which was too hard to file, so "plan B" (my plan!) was put into action - file the new crown race down to fit the cup!! This is an alloy crown race and I was able to file it, carefully, until it fitted the A9 cup without contact. I did notice that the caged balls do not run in exactly the right place on the crown race (there's a bit of wobble) but I decided to reassemble and see how it felt when complete. So the A9 cups were fitted to the frame using a fitting tool manufactured in my workshop - a very hefty piece of cast iron pipe about 10" long and about 1.25" diameter, with a separate brass ring about 1.75" diameter, and about 1" thick, machined on the lathe so that the hole through the middle was a comfortable fit over the steerer. The cast iron tube, with a little assistance from a hammer, was used to whack the brass ring, which was resting on the new crown race. It went nicely into place on the fork crown. (As an aside, before I could even start the job, I had to manufacture another brass tool to get the old cups out of the frame steerer tube - the 60-year old Myford still earns its keep).
With everything reassembled and the headset carefully tightened, I have now ridden about 100 miles and it's absolutely fine - no looseness, no 'notchiness' so the job's done and I'm grateful to SJS for their advice, as I don't think they could have anticipated this problem when they specified 27.0mm eleven years ago.
Now there is a post-script to this tale. I visited the cycle bits stand on the local market today, and while browsing (as one does) through an odd bits box, I spied a crown race. "How much is this" I asked. "Oh, it's probably part of a headset, the other bits might be there" was the reply. I rummaged for a moment, and asked if he'd got a ruler since it was certainly worth having if it was the magic 27.0mm size. The best that could be found was an ancient imperial steel tape, so that didn't help, then the stallholder said "oh, you can have it". "Thanks!" So I duly went home with a crown ring, wondering if it was 26.4 or 27.0mm.
Well, it's getting late and that's enough for now.
