Well yes, I had Octopus redo the calcs with some extra information and the heat loss was halved and only one rad needed swapping. Octopus has to tick some MCS audit boxes, but are open to rational haggling in effect.
After some observation including a couple of days that hit design days (-2C outside) I adjusted my weather comp to (50°C/-7°C, 25°C/14°C).
https://www.earth.org.uk/heat-pump-16WW-control.html#weather