Noisy buzzing alll night for months but now no hot water

Have a boilermate 2000 for the last 4 years, always buzzing noisily at night but working fine otherwise (after some initial teething troubles with the combi not firing and zero help from Gledhill who seemed to pass the blame always to the combi. Turned out the boilermate wasn't wired correctly to the combi).

Recently the hot water doesn't want to work. It starts off hot and then runs out after a few mins. Turning off the system seems to kick-start it but then the hot water stops. Central heating is fine.

I have no skills in replacing any of these parts myself. Does anyone know :
What its likely to be?
What it is likely to cost?
Any people in the Cheshire area that can fix these?

Many Thanks