Overheat cut out worcester 9.24 electronic boiler?

Had no central heating but hot water ok, boiler shuts down after 2 mins, found (eventualy) that the thermocouple top left U/S. Now have heating and hot water however an intermittent fault remains.
Overheat clicks out(red button at front) this happens usually when running a lot of hot water (bath) with the heating also on.
My conclusion, is either it is getting a message to hot, or the button itself is faulty? Noticed two wires going to another thermocouple? at the bottom of the heat exchanger could this be the issue? diverter valve seems ok. Lever moved to winter setting (is this some form of restrictor) pump ok set on highest flow?. Is there any "haynes type manual available". Thanks for any help.
Regards Roger.