Ravenheat CSI 85 A (T) Burner Cycling Constantly?

Hi all...

I've got a Ravenheat CSI 85 A (T) combi and shows the following symptoms.

It's fine with HW only, but when using the heating the following occurs. Starts at 1.5 bar and when the heating is turned on, it creeps up to just over 2 bar during the first five minutes. After around 30 minutes the burner starts cycling on/off every 5-10 seconds or so.

The Ravenheat fault finding says either ignition control board fuse (which it's not, as I've checked that) or the gas valve, but mentions nothing about the fan control board, which seems to be mentioned as a cause quite often. It's not the thermistor or PCB as both these have been changed within the last couple of months (yeah, am having good luck at the moment).

And no, I'm not trying to fix it myself, but would like to know if possible which part is required before getting the heating engineer in, as I can save a packet ordering it myself rather than through his company, plus it would be here when he comes, so only one visit to pay for!

TIA