Can anybody offer advice my central heating has started working when switched off, but when hot water is switched on it has a boiler in the kitchen and a tank and pump in the loft i have been told somebody had a simular problem fixed by a part on the hot water tank can anybody ether tell me what this might be or as the pump is running is it an electrical problem with the timer ?

We need more details on your system. Is this a system where you have a coil for hot water in the boiler that heats water to heat the house? When the system calls for hot water, the circulating pump for the heating system comes on too? If so, you need to take a careful look at the controls.