The thermocouple senses the heat from the pilot flame so that the main burner can safely open.
The pilot flame needs both a spark and the pilot gas valve to be open.
That requires the air pressure switch to be made which in turn requires the covers to be properly in place and the fan to be running.
It all begins with a call for heat - from the programmer / timer.
The thermocouple senses that all these things have happened and that the pilot flame is lit.
You may do best to call in a Gas Safe engineer - most of them are ethical but the odd one or two might be tempted to change parts on guesswork - at your expense.
Do not be frightened to challenge any diagnosis and ask why something is being changed - and if it does not fix the problem ask for the old part to be put back. It is VERY unlikely that more than one part will have failed.
Of course - most Gas Safe people are very ethical.
Good luck....
BTW - it's perfectly legal for you to mess with your own gas boiler but it's very dangerous for you to interfere with the gas-way. Houses blow up from time to time.
Pontious Pilot, September 2014
