Good news! I followed the black wire in the junction box by the valve along the twin core cable back to the roomstat downstairs. (The signal from the roomstat was coming in on the same cable) and lo and behold, after removing the front of the room star and unscrewing it from the wall box, there was a black wire dangling, with a bare end not connected to anything. And just next to it was another black wire, also bare ended, and also hanging loose, that had 240vAC on it when the CH was selected on, that dropped to zero when the HW was turned on also! Result! The two are now connected and insulated, and I now have a perfectly functioning Y-plan system again!
And a spare 3 port valve!
If the previous owners are correct, it has been like that for the last 14 years.
Thanks again for all the help. A simple wiring fault all along, with no signal to turn the HW off, as you suggested.