I have a heatslave 15/19 that exhibits a similar problem. In my case I can get hot water and /or heating by knocking the timer front panel. On doing this the relays inside operate and the boiler fires up. I haven't checked it out yet (much to my wifes annoyance) but my suspicion is a dry joint on the timer pcb. Hope so as a replacment is around £120.
With regards to the diverter valve mentioned earlier. Anyone thinking of changing this needs to get the part number from Worcester as I changed one once with a standard valve from Honeywell and it didn't work. Apparently the Worcester one has some of the connections reversed?
Best of luck
PS if anyone has found a bad joint on the timer module I'd appreciate knowing where abouts it was.