A corgi registered plumber recently changed the fan in my glow worm combination boiler, because the old one was making a noise. This is now fine, but since the plumber left the hot water doesn't work properly, the boiler fires up when you turn the hot tap on, then shuts down after a few seconds, then starts up again for a few seconds etc. The water pressure seems fine, and I don't think the boiler is shutting down because it's overheating.
If I turn the heating on for a while, then turn on the hot tap, the boiler sometimes stays on heating the water.
Any ideas what this could be, I assume it's some sort of bad contact that should be simple to fix?
The original plumber has twice promised to come round to fix it and not turned up either time, and now I can't get hold of him...