Problem with Worcester Heatslave 20/25 Programmer?

I have had three programmers fitted to my system for one reason or another. This one seems to be causing the fault below.
I first noticed a problem that the heating wouldn't come on at all. I found that if I meddled with the front panel switches i could get the heating on by first switching off the water.The engineer changed the diverter valve. This didn't cure things and Unfortunately we live in an area where we get quite a few short duration power cuts and whenever we had one the heating would then not come back on until we first switched the Hot water on and off again. We then had a wireless thermostat fitted, which, said the engineer, would negate the need to ever switch the heating/water off. Now we find that we can only have on one or the other and to get the Hot water on we have to switch off the Heating and Vice Versa. Anybody got any clues how we can solve this as it seems that this model is now discontinued! I guess that as we are able to get both hot water and heating albeit not together, the diverter is sort of working!(could it be anything else ?)