Thanks for the input, chaps.
Bit I hadn't quite worked out is why the 2 valves were on the same circuit. Or in other words why the HW & CH look to be on the same piping, if that makes sense!
But DP's response now clarifies this.
I'll sort this out at the weekend and let you know how it goes...
Hi folks, I'm in the same boat as a lot of some of the other people on this forum and happy to figure out where things are a miss, but hoping someone can maybe provide some indicative thoughts...