Thanks for all the feedback.
Yes, it looks like its a bit of a bodged install, with probably a number of factors causing my problems. I'll install a new head for the valve and will move the thermostat higher up the tank (would have missed that one, so big thanks).
Hi all,
My central heating system is working fine, but I've got not hot water running.
Previously, manually activating the Zone valve closest to the tank (Honeywell V4043) let the water flow fine.
It had got very jammed and I had cleaned it up with WD40 and a drop of oil, which freed it...