If you have a manual valve then you cannot have room stat as this would turn pump of which you need to run for when you have HW only in the summer.
You need to go one step further and get at least one motorised valve on the heating circuit, better still 2 one on CH one on HW with cylinder stat
As I said, I am aware of the issue. I can, in the summer or when I require instant hot water turn up the thermostat until I get the water I require. It's not perfect but for the moment it will have to do.