John Carroll

Oh yes, wait for @SimonD. When you were running on pump speed1 (6M) the boiler, just before cycling, had flow/return temps of 69/44, a dT of 25C, that boiler apparently has a minimum output of ~ 6.5kw so, by calculation, the flowrate is only, 6.5*860/60/25, 3.73LPM, which, with all those rads just doesn't add up except some big restriction somewhere, you then increased the pump head to 8M and no more cycling, the flowrate should have increased by just over 15% due to the increased head so it still looks as if the heat demand is quite low, maybe, when you get the opportunity, start with d.0 at 7kw, the boiler flow temp should then surely start falling and just increase d.0 (1kw steps) until the flow temp increases to its target temp of ~ 70C, this will then tell the flowrate almost exactly. (by calculation)

You could run on CH only, as above, pump on speed 3 (8M) and when the temperatures steady out just close the manual by pass and see what effect this has on the temperatures, particularly, the return. If the boiler remains running just keep reducing d.0? output in say 0.5kw steps until the boiler flow temperature just starts to fall, its quite easy then to calculate the actual system flowrate.