Thanks for your help, I'll hopefully get to it over the weekend.
Letting air out will also cause the pressure to drop, like letting air out of a tyre.Forgot to mention, all rads are full of air generally and I bleed them very often - assume that's a given if the pressure is dropping though.
Also seem to have a problem with the three port valve still - I can't quite work out the issue but the heating seems "weak" when the hot water is being heated. When the hot water goes off the heating picks up.
Unfortunately I played with the switch on the 3PV at the same time I killed the hot water so I'll have to test it again.
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