Much appreciated for the advice - and thanks for spending the time doing that.
The diagphram was a genuine Vokera one, but I have to say, I ran the heating for a while this morning (about an hour-ish, and played with the hot taps whilst it was on), just trying to figure this stuff out. Anyway, I was about to go out and thought I'd turn the heating all off, and I did so - then went to wash my hands and without thinking turned the hot tap on again - and oddly, even without the heating on - all was well - nice hot water.
I then went out for 4 hours, just returned home and thought I'd try a hot tap again - and voila! - its working perfectly.
How odd is that - unless of course there was air in the system somehow (I did need to top up using the filling loop after it had been run.
Thanks again - I'll keep my fingers crossed, and have a think about the things you've mentioned (it takes a while for my tiny brain to register this stuff and figure it out!)
Replaced the fan as per the service manual instructions. Followed to the letter.
Pilot swirls when electricity is switched on and what I think are swirls are seen when burner is on. Fan definitely runs at slow speed when idle and then switches to high speed when demand for CH or HW.