Thanks for the input guys.. just to clarify, the best way I can describe what I'm looking for is a dual inlet/outlet bypass valve and I definitely don't want to use TRV's after reading some of the problems people have had with then on single pipe systems.
Does anyone know of a UK supplier for these :?:
So far I've only found them in Germany and Sweden which is alright but I'd much rather source them from somewhere relatively local.
Any help greatly appreciated...