When designing an installation, with radiators sized for a flow temperature
of 70oC with a return differential of 20oC, you may permit the use of
smaller pipe diameters for the heating circuits.

In any case 22mm will be quite enough. There won't be any saving to had
downsizing to 15mm and I'm happy with 22mm rather than 28mm as it will fit a
pipe bender. I haven't calculated the radiator powers yet, so don't know if
70/50 will be doable in reality. They look to be on the small side to my
eye.

Christian.