What's wrong with my Vokera boiler/heating system?

Hot water works ok to programme but heating only comes on on the first timed option. After it has reached specified heat, the radiators go cold and the heating does not fire up again. The only way to get any heat after this is to switch off, leave for 10 mins and start again. Engineer fitted new time clock and PCB and now we have absolutely nothing and engineer is stumped. Any help would be appreciated