Potterton Prima 50 F main burner will not remain on?

I've got a twelve or thirteen year old Prima F which has run reliably until the last year or so. I fixed the usual cycling problem using new high temp capacitors about two months ago. Now the boiler ingnites correctly, but the main burner goes off after a few seconds. The boiler then re-ignites the main burner and again it switches itself off. Sometimes the main burner will remain lit and the boiler provides hot water and or heating according to stats and settings. When the main burner is igniting and then turning off after a few seconds, the boiler pilot light remains lit. Any ideas?
I'd just like to know what might be the problem so I don't look like a complete muppet when I get a corgi in to fix it.
Thanks

jnib
January 2009