Potterton EP2000 won't control central heating?

Hi all,

I have a Potterton EP2000 in my home and it operates the hot water perfectly well.

However, regardless of what position the central heating switch is in, the thermostat controls it regardless. The switch can be in the off position on the timer, yet if you turn the thermostat up, the heating switches on.

I'd really like the timer to take priority as there are times I do not want the heating on, and I'd also like it to come on before I get home from work so the house is warm!

Any help would be greatly appreciated.