Sabre 24 se red light from overheating?

Hi, Sabre 24 se compact boiler, heating goes on but cuts out after 20min when rads get very very hot, boiler keeps working if running hot water. Guessing its an overheating related safety cut out.

I have replaced both thermistors
I have removed and cleaned venturi pipes

Anyone know what would be the next best thing to try with this fault?

Appreciated any help given.