I was a little hasty in my previous post. Didn't have the boiler on long enough to check properly.
On the heating and water setting it runs fine, always has though. On water only it doesn't fire for long and the flow pipe gets silly hot and takes ages to cool back down again.
On the water only setting the zone valve closes the feed to the central heating so it has to take the 15mm route through the coils in the cylinder, through the gate valve, then 20 feet of 15mm where it taps back in to the return from the boiler.
Apart from the first 5 feet where it goes down from the cylinder cupboard, that 15mm return is horizontal under the floor boards.
So my guess is there's a blockage somewhere on that run.
Could be.
Blocked coils - it's a twin coil cylinder so both would have to be blocked
Gate valve broken and stuck shut - but the spindle does turn and stops where you'd expect it to
Blockage in the long horizontal run under the floorboards - under floorboards and tight under the end of the bath (tap end of course)
I'm ruling out a blockage in the 22mm flow pipes and through the air seperator because that would mean the central heating would be affected too.
My guess would be the long horizontal run, probably just after the elbow coming down from the cylinder cupboard. Blockages love a bit of horizontal pipework smiley-face-shaking-fist
It's worth adding that the spring clips in the zone valve were broken up until I sorted it a few days ago so it was stuck open, so the main f/r water always had a nice easy run through the c/h pipework even if the timer was set to water only. The tenant did mention it about 18 months ago but said they very rarely use the heating or hot water so wasn't a problem, so being a lazy tight arse I never sorted it until I moved back in.
!
!
(I believe it was one of your top pips) foto 
