Hi
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!)
cheers
neil.