Right, been up in the lofts today and had a good look at how everything is
plumbed. Can anyone advise if they would go with zone valve and keep a semi
gravity system - or convert to fully pumped (and how - placement of valve
etc)

Current system is setup as follows.

Boiler downstairs utility - two 28mm pipe flow and return come out of the
boiler, through the downstairs loft and upwards to the main loft upstairs
(cylinder). One pipe enters the top of the cylinder coil via a T piece which
then carried on the the curved overflow pipe which is over the
expansion/header tank. The outlet of the expansion tank goes to the bottom
of the cylinder coil via another T piece and then connects to the pipe back
to the boiler. Thus the two gravity HW pipes from the boiler also serve as
the header refill.

In the downstairs loft there is a T piece fitted to the 28mm pipe taking a
22mm feed which is connected to the pump which then connects to the heating
flow circuit. There is also a T piece fitted to the main 28mm return pipe
which is the return from the heating system.

For ease of wiring any valve etc would need to be fitted in the downstairs
loft near the boiler (rather than near the cylinder). - but as I see it this
would then block the heater tank feed, on either a 2port valve or fully
pumped system.

Is it possible to add either a 2 port valve, or convert to some sort of
fully pumped system to the HW without serious modification?

Let me know if you need any more info.

thanks