Worcester 24i RSF combi boiler ?

Could be something simple like the hot water sensor that's the red thing with 2 wires on. that's the one at the front of the boiler. or it may be the PCB that's the expensive part and what you pay the engineer for his expert diagnostics. electrical parts for boilers ar none returnable so do you feel lucky a pcb is the best part of �200. very difficult to say exactly whats wrong with out tests.