Seb wrote:

The gas valve (Honeywell VR4601T A1046) has a pair of terminals
towards one end, and a group of three terminals at the other end (*).
I guessed from the layout (but it's only a guess) that the pair are
the terminals for the pilot valve solenoid. I found 240V across these
when the boiler was trying to light. From this I concluded that it is
the gas valve, and not the PCB, that is faulty.

If I remember correctly from my Baxi Eco 80 (long gone now thank God, a
beautiful Vaillant EcoTec Pro now doing the duties) you can test the gas
valve by putting a manometer on the test point and watching the gas
pressure rise as it opens. You can either make a manometer, or BES have
them for a few pounds.