I hope that somebody can help, I live in the West Midladns, UK and have an issue with loss of pressure from my hot tap in the bathroom.

I turn the tap on and hot water comes out then after about 5 seconds it goes off and I hear a knocking sound and you can feel on the sink a slight vibration. If I turn the tap open a bit more i.e. 2 turns hot water will come out then.

We have a combi boiler system.

Can anybody help with this please?

Sounds like a loose washer in the hot tap. Could be something in boiler mixing valve but try this first. Turn off water , remove handle and stem of hot tap and tighten the washer screw.