Glowworm Ultimate 40FF Multifunction Control Overheat Reset Button trippin ?

Multifunction Control Overheat Reset Button on Part no 313980 at base of

Glowworm Ultimate 40FF Central heating Boiler Serial Number of boiler 960029278

has tripped out 3 times in the last week.

The boiler starts up straight away when I reset it then runs ok for a day or two?

I am an electrician and have the boiler serviced Annually by a Corgi registered Central heating Engineer.

Before I call anybody in can you suggest what the fault may be please?