Pilot switching off on my Baxi Solo 2 (50 RS)?

I have a Baxi Solo 2 boiler, model 50 RS. The problem I'm having is the pilot light switching off. I've tried to narrow the problem down, and made the following observations.

If I light the pilot, and the system doesn't demand any heat, then the pilot stays lit indefinitely (over 12 hours anyway).

If the system demands heat and the main burner ignites, and I then interrupt the cycle after just a few seconds (by turning the main control knob on the boiler to '0'), then the pilot remains lit after the main burner turns off. I tried this for 10 short cycles and the pilot remained lit throughout.

However, if I leave the boiler to run through it's complete heat cycle, when the main burner turns off the pilot remains lit for about 1 second, and then there is a click and the pilot turns off. This doesn't happen every time, but most times anyway.

When lit, the pilot flame looks good, and fully envelops the thermocouple.

There ends my knowledge!

Any suggestions will be gratefully accepted.

Thanks in advance.

Russ.