EP2000 Potterton control box?

Hi, we have just replaced the battery in our unit, but still have the problem we were aiming to solve - which is when we try to put the hot water on (but no central heating) the unit flicks the H/W on and off very quickly and the C/H and will only STAY on, if you then put the central heating on too. I don't often want the C/H on, so what is the solution anyone, as I can't afford a new C/H system just yet.