Any idea why my EnergySaver Glowworm (80) boiler shuts down the flame?

Boiler model number: GC 41 319 84

It lights the flame but the flame goes out after a few minutes. There's a click sound which suggested to me that the contol unit is physically shutting down the boiler. The demand light stays on but the flame is out.

The boiler did recently have a condesate drain blockage but that's freed now and I've even left out the drain plug on the bottom right, just in case (handy use of a bucket). The condensate drips into the bucket nicely so I don't think that is the problem.