Potterton Puma 80 fires up then stops?

potterton puma 80?

I run my central heating and the boiler ignites for about 20 seconds then stops. it will not run again for a long time. but if I run the hot water the boiler ignites and runs ok until I turn the tap off the boiler runs for another 10 seconds then goes off. 10seconds later the pilot light goes out. I believe it is a stat. but which one,This happened at the end of last year but everything started to work ok again. now the problem has returned can anybody help