If you find that the water dribbles out of the hot tap (and out of an overflow), but flows out of the cold tap ok, the problem could be a non-return valve on the boiler. It is located on the top right hand side of the boiler whilst looking at the control panel and is there to let air in when draining the water. Remove the two plastic tubes. Warming with a hair dryer softens the plastic. Then unscrew the collar holding the plastic T piece onto a metal tube in the wall of the boiler. If you can see through the valve , it’s knackered. Replace with the new valve, not forgetting the O ring. The part number (for a C3402 is 34151-03) can be obtained From Truma Technical tel 01283 586020. The part is supplied by Miriad tel 0123 586060.
Info from P Rowbury