Un-demanded heating

[ChrisinShropshire]

I have a Worcester Bosch Greenstar 30 CDI with a Honeywell T4 hall thermostat.
Occasionally, we get undemanded heating, to the extent that the set point was 17°C and the hall temperature was 23°C. We called out Worcester Bosch who said there was a false feed (approx 57V) from the heating zone valve and fitted a RC device. The voltage dropped to <1V.
However, the problem has not gone away!
So I investigated changing the zone valve. I read on the screwfix Q&A that the White cable is not used in our application but might be the source of the false fed if not properly terminated. However, I found the wiring in the 10 way connection box did not correspond to the honeywell S plan wiring diagram. Also the wire colours differ and the white cable was not terminated.
Is it possible/likely that the zone valve is the problem and what are the consequences of the strange wiring in the connection box, please?

I think I need to get a heating technician in.

 

[Madrab]

With an S plan there shouldn't be a valve with a white wire! A valve with a white wire is a 3 port mid position valve that is used on a Y plan system.

Can you post pics of the valves and their relative position in the system?

 

[Exedon]

We have had a number of replacement honeywell 2 port heads with the white wire.
Instructions just say to terminate safety when used on S plan.
The white wire is live when valve closed used on C plan.