Thanks Scatman. Hit the nail on the head. Fitted a new Ignition Control Box (£112) earlier this evening. problem fixed.
Saved me a few quid as I was going to go for the main board first.
Cheers
many thanks Craig51 for you reply.
Checked the pressure switch and fan work ok. previously checked venturi by blowing in tube (not easily done) However yesterday had an error 3 come up which is the flue fan. I couldn't clear it until this morning. Think it was a dirty contact on the pressure...
I have a Vokera Linea Max (6 to 12 years old?). The boiler has recently started this fault. The boiler jets fire but go out within about 3/4 seconds. It does this continuously once it has got up to temperature. Reading the manual it appears to do this once the 120mA modulation phase is entered...