Hi John, thanks for your reply
The boiler is a Worcester System boiler so has its own pump. Each manifold also has its own pump but as they were originally paired to a GSHP there is no TMV as the water from the GSHP was generally cool enough for the UFH circuit. The cylinder is “always on” and controlled by a cylinder stat.
A low loss header, from what I’ve read about them, seems to be the perfect solution for this situation but of course I’m always willing to be pointed down the road of righteousness!