Intermittent fault on the hot water. Sometimes it comes on but when it doesn't I can sometimes get it on by turning on another hot tap which fires up the boiler. No problem with the central heating. I've had a new diverter valve and new circuit board(?) but still have the problem.

Pat Mannakee, September 2017

Possibly the demand valve, which senses when hot water is needed.

ccsa, September 2017