I have a Potterton Puma 80 combination boiler and when I go to light it the pilot light goes out on release of the button. The thermocouple has been changed and everything has been cleaned, the gas has been purged and all contacts are sound, the initial pilot flame is sound and it doesn't matter how long the gas button is depressed or how slowly you release it the light goes out. Please help, thank you.
Have you checked the pressure in the system. With the work that's been done it may have lost some. If it is too low the pilot won't light for safety's sake.
Have a look at the manual and see what the recomended pressure is cold and turn on the cold filling loop tap until the pressure gauge reads the recomended pressure. Your pilot should then light.
Pottertons are currently being investigated re their boilers (log in to Watchdog website) there is a problem with their circuit boards, although the Potterton company will not admit it, However thousands of Potterton boiler owners have experienced the same type of failure. When a plumber is called to a boiler they now ask "What type is it" If the reply is "A Potterton" they automatically turn up with a circuit board