I have an ST6400C Programmer which I think is malfunctioning. Quite simply, the heating will not come on unless it coincides with the time that the hot water is on. Thus, as our hot water goes off at 8pm, we cannot heat the house unless I manually override the hot water settings, and swith the hot water on again. When the heating comes on, the heating light comes on, but the hot water one does not. I have turned the thermostat up to ensure that there is no problem there. Any idea what is going wrong?
Thanks.

Follow this link and make sure the thermostat is programed correctly.

http://content.honeywell.com/UK/homes/Technical/ST6000/ST6400Cuser.pdf

If this fails to correct the problem it could be a relay for the heating side of the unit. Or the thermostat itself.