Suprima 50 HE No lights unless Gas Valve disconnected?

Hi,

I have a Suprima 50 HE boiler that decided to stop working yesterday. When I got to it, all 3 lights were on and so I followed the usual instructions and reset the temperature control knob. All goes well and the boiler fires up and I walk away.

About 10 minutes later (or so), we notice that no heat is coming through so I check the system again. This time no LEDs are lit at all. My corgi plumber friend checked the system up to the PCB and we could see mains power getting to the PCB fine. Diagnosis... Faulty PCB.

So, today I pick up a new PCB, fit it, and the same issue is present. I checked the harness and the rest of the system and all looks well. As I was pulling out the harness connectors from the PCB, I spotted that the system came into life as I disconnected the Gas Valve. The green light indicating Mains present is lit, and the middle green led flashes. The system tries to start up for a few seconds, and then stops. This process continues. I shut it down and started looking for help as my plumber friend is stumped.

Hope someone has an idea :) Thanks in advance!