My Potterton Envoy keeps switching off. Can you help?

I hope some-one can help me; I have a Potterton Envoy 40 boiler which starts, gets established on the flame signal, and then the burner goes off for no good reason. The flame signal is not being lost because I do not see either the flashing 'F' or the 'C' on the display. The 'call-for-heat' signal is still on (I have measured it with a volt-meter).

It keeps this up until it refuses to try again due to 'rapid recycling'.

It keeps doing this for between 30-60 minutes and the amount of time it stays on grows from several seconds at the start to several minutes, then it just stays on. Again, for no apparent reason. Nothing has changed and the overall time and number of times it goes out and re-lights seems random.

I'm beginning to think it is the control unit (AKA, the PCB). This is so expensive you wouldn't believe it, so, any suggestions would be greatly appreciated.

Thanks