Intermittant Boiler opeartion?

Good morning,

Friends of mine have a Vailant Turbo Max 828E, they say heating apparently works fine however when they turn the hot water tap on they get luke warm water.

I have had a quick look, the boiler fires up when DHW is demanded but shuts down after a few seconds; it then re-ignites after a few seconds with the hot tap left running then shuts shuts down again - it keeps cycling like this. Would appreciate some guidance to what it might be and where the component is located on the boiler.

Thanks in anticipation.
kind rgds
Mick