It does indeed sound like the overheat stat has failed, and shouldn't cost
much to have replaced on spec, even although it appears to work sometimes
- like when the engineer calls.

But you could just turn the boiler down initially to perhaps 80C or so -
it doesn't need to be full up for just heating the water.

--
*The early bird gets the worm, but the second mouse gets the cheese *

Dave Plowman London SW
To e-mail, change noise into sound.