Glow worm micron 50ff boilerstat?

Hi, I have a Micron 50 ff which seems to work perfectly in that the burner comes on when required and heats water for hot water cylinder and central heating. However when it gets hot the boiler stat turns the burner off while the pump continues to circulate the water. the problem is that it takes a long time before the burner comes back on, and the water circulating the radiators is by then only warm. eventually the burner comes back on and the circulating water starts to heat again. I.e. there seems to be a long delay before the boilerstat switches back in again.

Does anyone know if this is normal or have an idea of possible cause, or is areplacement of boilerstat a likely cure. I am an electronics engineer and have done most of my own plumbing so would be confident to tackle such a task.

Many thanks

Bob