Flame failure device keeps operating?

I have a Glow-worm Space-saver 40B MK 11 boiler.

As the pilot wouldn't stay lit, I have recently (last month) replaced the thermocouple; thermocouple re-router; pilot burner and eventually the gas valve (Honeywell).

Everything has been working fine, until yesterday! The pilot remains lit, but when the boiler has fired and after about 5-10 minutes, the burner and the pilot both go out, with the flame failure device operating.