How does this clock work? Are there little fingers on it that trip the switch? Or should there be? The Baxi Web sites don't show much detail about the timers. If it's electromechanical, the little trippers may be missing, or not set properly, or the switch may be broken. In any case, the timers are optional, which means they can be easily replaced. I don't know if this
http://www.alertelectrical.com/Grasslin-Timers-And-RDL-Meters/Water-Heating-Programmers-Timeclocks/1-Channel-electro-mechanical-timeswitch-GRASQM1.asp is your timer or not, but they say it's "
suitable for switching "Combi" boilers", so it might be similar to yours. It's under L30, including VAT.