"Bal" wrote in message
oups.com...
I have just replaced the Thermocouple and it has stayed up all weekend
and the same problem has started again this morning.

The boiler is a Glowworm Fuelsaver 40B MkII.

IIRC the 40B is a balanced flue without a fan and has a permanent pilot.
It has an overheat interupter stat in the thermocouple circuit.
Based on this the problem is either no/low water flow rate (does it get
noisy?) or a failing thermocouple/poor connections in the circuit.