Thanks for all the tips guys - I'm not so sure it is a wet issue thoough, as the bike is either parked in a car port or a garage - it might get a few drops every now and then in the car port but not a soaking.

Temperature seems more likely: following my successful 2pm start and ride earlier (bike didn't start at 11am), I parked up at 3:30, started it again at 6ish (no problems) tried and failed to start at 8pm.

I like the sound of CCA, had a quick scan and it seems in line with the symptoms (lower revs from the starter motor at colder temperatures). I'll call the garage to see if they tested CCA. I'm tempted to buy a new battery if the garage didn't test. How does that sound? However, that doesn't explain why it cut out after starting on the very cold day (before calling the RAC) when it first wouldn't start.

Pleiades, the pump always primes and I've taken to waiting until everything on the dash has settled down before starting.
No fault codes have appeared on turning the ignition on, I tried and failed to get into diagnostic mode (will double check and try again). When it's cut out mid ride, it always starts immediately - don't even need to put my feet down.

Thanks again.