There are a number of answers. Firstly is there 240v at terminal 4 on the PCB? If not, fault will be external to the boiler.
If yes:
Look at the diverter, with heating selected on, do both microswitches on the right hand side of the valve 'make'?, if no, possible issue with the servo valve above the water section, try pushing the servo valve arm (behind the servo) down on the right hand side, see if the microswitches make then.
That should be enough for you to go on for now.........
Carefree
October 2010