I checked the wiring again and the problem is solved - it's so obvious and
rather embarrassing.

Basically, the original test to determine which two pins controlled the CH
and HW was cocked up.

I originally thought pin 3/brown controlled the CH and pin 6/blue did the
HW - wrong its the other way round!

With pin 6/blue wired into the room stat and pin 3 left alone this time
everything is working properly. The annoying part is that I used brown wire
to extend the ON wire to the new stat so it now looks a bit of a bodge as a
blue wire is connected to a brown one.

So Mike, the wiring is straightforward, just avoid the mistake I made!

Thank you to everyone who helped.

Jules