Worcester 28i Combi Boiler, not hot water?

I have a Worcester 28i, the central heating works perfectly, but there is no hot water when I open the tap, unless the heating is on, in which case there is a little hot water.

If the heating is turned off, the boiler does not fire and so does not give hot water.

Please advise, what you think may be the problem or how I can resolve the issue.

Thank you