My hearing turns off when I run the hot water

If your heating is on and you run your hot water, the hot water that normally flows to your radiators is diverted to the plate heat exchanger to heat the water for your taps. so your radiators won’t heat if your hot tap is running. When you close the tap it is then diverted back to your central heating automatically. If this does not happen you have a faulty diverter valve.

I take it the boiler is a Combi. It switches to hot water when you run a tap, you will not get heat and hot water at the same time but it should switch back to heating when you turn the tap off.

Yeah as above combi's can't really do Heating and hot water at the same time so will always priorities hot water. As soon as you shut of the tap if there is still heating demand it should switch straight back..

This way of operating is a standard in all combi boilers on the market past and present. When there is demand of central heating and at the same time there is hot water demand at the taps, the combi boiler prioritises the hot water demand and stop providing heat to radiators.