OK, but if the UFH pump wants to move 15 litres and the Radiator pump wants to move 10 litres - but the Radiator pump is switching on-and-off then what does Nick try to match with the Charge Pump speed: 15? 25? 20 as as halfway house?
I just can’t understand how a big, 2-pipe Buffer Tank piped as a ‘short circuit’ between Flow and Return is the right choice for Nick’s installation, whereas I can understand how a smaller, 4-pipe Buffer Tank would isolate the multiple pumped circuits from each other - something like the black tank shown in this other thread (which appears not to need it, since it only has a single emitter circuit).