Combi boiler - hot water okay but blows fuse when heating swtiched on?

I have a Worcester-Bosch 28iRSF. Having not used heating for many months, recently switched on and blew the fuse. Having replaced appropriate fuses, the hot water is working but switching to heating immediately blows the fuse again. I have hand-cranked pump in case it was stuck but to no avail. Wiring appears okay but haven't been able to do full electrical testing. Instruction booklet suggest replacing PCB but I'm not convinced yet - any ideas on other tests I need to carry out?