Potterton Netaheat balanced flue - pilot light?

The pilot light would fire and all the cental heating would work fine but just recently the pilot light would fire then go-out and refuse to operate leaving us with no central heating. Could it be the thermocouple and if so any ideas where I could obtain a replacement part.
By the way it is an electronic 16/22Kw and for the record it is aprrox 22 years old.

Thanks Guys