Worcester 350 boiler cutting out?

I have a worcester 350 Combi GC Number 47 311 13 about 12 years old.

This boiler is always been first class but I have a recent problem.

When the water has been heated say for a bath or shower, as soon as the water stops the overheat stat operates and we have to wait foir the boier to cool down to re-set the stat. The central heating seems okay.
PLEASE CAN YOU HELP.