Thanks for all your help - very useful indeed.
It seems that a separate zone, with it's own controller, is the way to
go.
I'm a bit wary of the systems with only a thermostat on the return
path, as the water entering the UFH pipe will be hot!
Other systems I've seen advertised on the web have a CW blending valve,
room thermostat, and a pump. I'm curious as to why the UFH needs its
own pump, when the boiler has a perfectly good one.

Nigel Frost