One and all, thanks for all the information - especially that this
convection behaviour is not confined to just me. As a DIYer (fast
becoming C/H engineer) I'm surprised that the 3-port mid position
valve is sold seeing as it has the Port B motor drawing current (with
no CH or HW demaand) issue aswell as the convention waste of hot water
issue. I wish I'd gone for a twinzone double 2 port valve system
(lower failure rates of the valves and closed if not energized) but my
3 port mid is now working (well as well as it's designed to).

I've seen some posts about W plan 3 port diverter systems not being
the best system and users changing to Y plan. So not convinced on this
strategy.

Me thinks I'll either put up with the system as is OR probably use the
old Honeywell 2 port valve (I was upgrading my old system without room
or cylinder stat and inability to have independant CH only by using a
Simp-L-Fit packaged BiFlo solution from Invensys/Drayton). Anyway, I
fitted an ABV so I guess I can install the old 2 port valve in the
pipe between the Port A of the Drayton 3 posn valve and the cylinder -
ideally only being energized (i.e. open) when the pump/boiler is
running and the 3 pos is in either A or A+B? Not sure I can figure out
the wires for that (only 3 used to be connected - L, N, E), so maybe
I'll just plug these into the Pump/Boiler terminals (OK the 2 port
would be open unnecessarily with a CH only demand). When the 3 posn
fails then I could buy another 2 port and switch to a twinzone system
(which I wish I'd gone for now I know something more about this
subject). Anyone any ideas on ideal wiring of my 'stop convection
wasting heat energy' 2 port valve?

Thanks for all the help so far, very useful