2nd question. Heating not working right on my worcester 240 combi RSF?

Heating goes on then after a min or so the boiler goes off and on with a clicking sound like its trying to start up. After 15mins or so the problem goes but it starts again when heating needs to go on with room temp. we have no problem with the water, and the boiler fan is working. anyone out there to help please? Thanks RJ for your reply but it only started playing up this year. some boiler man said the bit coast £21 but i forgot the name of item he said so hoping someone here will say the name that will rings the bell. he said i could fit it but they charge £130 so £21 is better then £130 as not playing with the gas