Thank you to both namsag and Soggy_weetabix.
In fact it was serviced last September for the first time in 13 years - quite possible some dirt got shifted and ended up where it shouldn't?!
I'll certainly check out the injector first but, if that doesn't do the trick then at least I can see...
I'd appreciate help with 2 things:
1) Confirmation that I've diagnosed the problem correctly
2) If I'm right that I need a new GCV, where do I get one?!
Hold the Start button in and press the Igniter button: the pilot lights.
Count to 20 (or 30) seconds, release the Start button, pilot...