heating will not urn off

Honeywell 2 port zone valves?

Does the heating valve open and close when the room thermostat and/or programmer is not calling / not calling for heat for heat?

You can tell if it's open as the manual lever at one end of the valve can be moved without any resistance/ When it's closed the manual lever can open it and you will feel considerable resistance from the return spring and gears.

If it is operating OK the probable fault is the micro switch in the valve is stuck closed keeping the boiler firing.

Agreed. The OP has an S-Plan system, where the programmer and room stat control the CH zone valve and the programmer and cylinder stat control the HW valve.

Both valves have auxiliary switches which close when the valve is fully open. These have a permanent live feed, and switch on the boiler and pump when either or both valves are open.

If a valve sticks open, or if its auxiliary switch welds closed, the system will operate regardless of what the programmer and stats are doing.

So that's where to look!

This sounds similar to my earlier thread, entitled: Honeywell Boiler Wiring. A lot of your debugging information may well be in there.

Good luck! The good folks here know what they are talking about, and have helped me considerably.

There are some similarities in that, in both cases, a zone valve seized in the open position can cause the HW and/or CH to run when it shouldn't

- but when you get down to detail, there are a fair few differences between a C-Plan and S-Plan system.

Hopefully, the S-Plan-specific answers provided by others and myself will have been some help.

Is there any progress on this problem, Andy with with no 'Shift' key? It reminds me of Archy and Mehitabel.

He must have a shift key - he used a question mark.

Good point.