Firstly my apologies if this is the wrong place to post this - I'm new here and couldn't find the answer in any other thread. Mods please feel free to move it as necessary.

I am a user rather than an installer, but took an active role in the installation so I could understand any problems that arose.
I have had an evohome system fitted and running successfully for ~6 years now.
Following some recent maintenance it appears to be stuck in 'learning mode' on one zone

The installation comprises 12 zones with 5 BDR91 relay boxes controlling 5 valves

Valve 1 responds to demand from any of the radiators in zones 1-9 fitted with Smart Radiator Valves
Valves 2,3 and 4 respond to demand from 3 DT92E thermostats
Valve 5 responds to demand from the hot water system

Recently I had comms problems with valve 2 (Zone 10) - it kept dropping out even though the batteries were new. In order to maintain service I ordered a replacement DT92E/BDR91 kit to swap out the faulty DT92E.
The BDR91 is in a challenging place to access so I decided to leave it in place and bind the replacement DT92E to it.
I deleted the zone associated with Valve 2 in the controller.
I reset the stored data in the BDR91
I successfull bound the new DT92E to the old BDR91 and created a new zone in the controller - so far so good ...

The controller can see the new DT92E and it shows the correct zone temperature
I added a new simple schedule to the zone for all days of the week (typically temp between 15 and 18.5C)

Once everything had settled, zone 10 radiators came on and the temperature in the area increased dramatically to ~25C before the radiators started cooling
subsequently they came on regularly throughout the day over a period that now exceeds 6 days.

I assume this is 'learning' as the heating system is currently set to 'off' with only heating on.

Today the temperature cycling has once again hit 27C and my gas bill is clearly increasing day by day!

Has anyone else seen this sort of behaviour and have any idea what is happening and how to fix it?