dont know if you have sorted it i know it was a while ago but the part you need is the main pcb part no.130331 about £112 just done mine and got the part from ebay for £50 works fine now
the make/model Vaillant VCW GB 242 EH
the problem
i had a standard timer switch fitted with the boiler controls ie. off timer 1 timer 2 and constant positions about 6 month ago i put the controls to the off position and the CH wouldn't shutdown so to shut it down i have to press the hotwater...