Thanks for the replies guys!
Spoke to a local Plumber as well today who owed me a favour from a few years back. He said he would be looking at the 3 port valve as well. He told me how to partial drain system, which I did, and replaced with a Honeywell V4073A.
The physical fitting went...
Would like some advice, before I finally section myself......
I've lived in house for 5 years and always had HW programmed morning and night, but CH on programmer set on Constant. I would then use stat to control temp along with TRVs.
Noticed in the past few days house has been a lot...