Quick update - I was fortunate enough to secure some help from an old friend of mine who visited earlier today. I'm not sure I understand how this has worked but he tells me he has replaced the diaphragm and the problem has been resolved.
I'm at a loss as to how it was working fine for CH but...
@DP thank you for following up. I haven't abandoned the post (nor the problem) but being back into the working week means I have very little time to work on this. I will update you as soon as I have anything worth saying but for now all I can tell you is that the problem is still there and I...
Huge thanks to @ianmcd - you are a real star and I cannot thank you enough for all your help and time with this. I tested the diverter valve and it appears to be working. I also tested the flow switch sensor with a magnetic screwdriver and that also seems fine. Sadly still no HW on demand...
@ianmcd
Ok so with CH off and hot tap open, nothing gets hot. The boiler doesn't fire up at all like this.
If I turn the CH off, hot top open and override the flow switch then I get hot water no problem.
Thanks @ianmcd
Can I check - when you said 'when you run a hot tap does one of the CH pipes get hot ?' did you mean overiding the flow switch so I get hot water or just open up the tap and let it run cold?