At the start of the day I had taken the guy at his word that the system was probably sludged up and had already bought the stuff when I bought the replacement 3 port valve. Figured I could no harm in using it now I had it.
For completeness, heres the diagram I was using - based not he fact that the wiring for the stat suggested 3 wires would indicate a Y plan set up.
www.electriciansblog.co.uk/wp-content/uploads/2012/02/Y-Plan-Biflow-Wiring-Diagram.png
As stated, C and 1 were swapped on the stat.
oh ok - so you think the 3 port was faulty at the outset and it wasn't the stat that needed changing. They are simple bi-metal strips by the looks of it - so not much to go wrong I suspect. If so that makes me feel slightly better.
While I had the system drained I did add some Sentinel x400...
I think i have solved the problem (and a bit annoyed).
I took off the timer panel and with the help of some wiring diagrams of the various units involved (and not helped by the birds nest of wires left by the original installer) I traced the wiring back. When the engineer fitted the new...
So recently my hot water was getting excessively hot (was way over the thermostat on the hot water tank setting). Engineer came and replaced the thermostat on the tank.
This had no effect. Still got very hot water. But after he left I found the Radiators were not coming on. Manually setting...