On Monday, November 7, 2016 at 11:37:17 AM UTC, Roger Hayter wrote:

But it must have some sort of flame
detector?

Yes - it does it via the ignition lead. The old red ones are a known weak point but mine has been replaced by the later white one.

However if that was faulty I'd expect the drop outs to happen randomly. In practice, once the boiler gets going, it runs fine until the it reaches the boiler 'stat limit. Then it mostly won't restart, going through three ignition cycles then locking out.

It may well be the PCB (again) but I'm wondering if the gas valve has separate ingnite and run coils and the run coil is failing when hot.