Thanks,I get your point about the bypass.
Saying that, I thought a one pipe system was that,one pipe flowing around the system with tees off the same pipe to each side of the radiator? I can see the bypass being needed if the flow went through the radiator and back out to the other side ie (the flow was cut), this would mean when one rad was turned off the circuit wouldn'd circulate.
Confused :D
If I go with one pipe that means quite a bit of alteration to the two pipe system I have already installed,I'm thinking the return I have fitted wouldn't be used apart from the last leg on one rad back to the boiler?
Thanks