MORCO feb 24 Boiler heats ok but locks out when temp reached?

I have a fault on my Morco feb24e boiler central heating - it's failing with fault code (no flashes) which says problem with 'lack of gas or ignition/Flame supervision faIlure/Burner on but flame indicator off/ Minimum burner pressure too low.
If you reset the code by switching off, the burner fires up normally with a good flame picture, heat happily to whatever temperature you set it at until the system reaches temperature, then switches off as you would expect. Then after about 5 minutes off the fault re-occurs - the boiler locks out, no flashes. If you reset immediately the boiler strikes up again and will heat again until it reaches temp again but the same thing happens again. The re seems to be no problem with the flame or gas supply when burning, and the hot water runs for as long as you like with no problems - so I have eliminated gas, flame sensor electrodes etc as a possible fault. The flame failure electrode connects directly to the PCB. And clues? new pcbs are 150 quid plus so dont want to buy one if i dont have to - Thanks!