Glow Worm compact 100e no hot water?

Hi can anyone please point me in the right direction, I currently have no hot water, the boiler does not kick in when I turn a hot water tap on, the heating still works.
Before the hot water went totally taps down stairs did not get hot water unless I turned a tap on upstairs to get the boiler to kick in, to run a bath we had to run the hot water tap slowly to get really hot water or it would be just warm.

anyone know which part has gone? and is it easy to replace or should I get a plumber in.

Thanks

Pete