CH Boiler (Ideal Mexico RS65) pilot light won't stay lit?

The pilot light of my aging (but previously very reliable) Ideal Mexico RS65 CH boiler will re-light, and flame is blue (no yellow) but looks a bit on the small side. When main burner cuts in, the gas valve very quickly closes - presumably because the pilot light isn't generating enough heat for thermocouple to keep main valve open - and all flames go out.
Assuming that the underlying problem is with the gas supply to pilot:
How do you clean pilot light/clear jet if blocked? or is it more likely to be gas valve itself/something else?