no hot water

I have a baxi 105e combi boiler and the only way to get hot water through taps is to put central heating on for a while. What is the fault and how to fix it as this can't be right

I don't know the detail for your boiler, but the basics are:
A combi incorporates two water circuits. That is a longish one including all the radiators and a shortish one which includes a water heat exchanger.
A diverter valve operates to open one circuit and close the other.

Hence you cant have both central heating and hot water at the same time.
So my opinion is your problem is related to the diverter valve.

When hot water is on and a hot tap is opened, there is a drop in pressure on one side of a diaphragm, the higher pressure on the other side pushes the diverter across and also triggers a switch.
These diaphragms seem to perish which results in insufficient movement of the diverter valve.

probably the diverter valve, call an engineer

Seeing as your so brief I will comment by the same rationell. I thinks it no too right tooo.

Divertor.... change it. Sort your o rings out yourself. If you cannot explain your problem properly, I cannot be arsed to describe a repair. IDIOT.

Mr. W.

wtf. bit harsh.....?