Just to update:
I went to County Cycles in Crosshands today, and had what they call a comfort fit (£75). They also offer a performance fit (£150); I'm not clear what the extra is, but for what I was after, apparently it was unnecessary.
Some chatting about what sort of riding I do (fun/fitness/freedom... more head up enjoying the view than head down watching Strava segments); my general health and injuries; my flexibility (high); measured me, measured my current bike.
Turns out I'm a bit lop-sided for no good reason, and my current bike is very much the right size/fit for me, which is nice. Adjustments limited to a tweak of cleat position and saddle height to take a little stretch off my left leg, otherwise I've got everything lined up pretty well already... which is also nice.
The process in general was calm and methodical and he made sure I was understanding why he was making the (small) changes.
Also included some general discussion on what bike I'll be looking for next year, followed by a sit on and gentle attempt to sell me the one bike in the shop that not only fits me but also the type of bike I'm after (relaxed road)... at around a third of what I said my price range was, so not really quite what I'm considering at all... he did show me the lead times if I wanted the same thing with a better groupset - September '21! So maybe not so pushy to suggest the one they actually have... however I'm not in a rush.
Oh, and the price of the fit is refundable against buying a bike from them.
Overall, it was useful. Although I didn't need a lot changing or anything, it has made me much more confident from knowing that where I am is a good place, and therefore understanding the sort of fit/geometry I should look for with my next bike.