To put some numbers against it. I used the MCS Heat Pump calculator spreadsheet to do a heat loss calculation and radiator sizing analysis, I’m confident that the methodology was followed correctly and the correct data entered.
The spreadsheet produces a heat loss of 5.3kW @ -3.9C, this requires a flow temperature of 53C.
If I re-run the spreadsheet with the actual temperature we keep our rooms at its a heat loss of 4.7kW at -3.9C, requiring a flow temperature of 48C.
Experimental data is that heat input required is actually around 3kW at -2C and that heat can be produced at a flow temperature of 35C.
During the recent low temperatures we had -6C at night, rising to a little below 0C in the day, total produced heat in 24h was 75kWh for an input of 24kWh (includes defrosts). We let the internal temperature drop by 2C or so overnight.
So the actual heat loss is about 2/3 of the calculated heat loss, and the actual flow temperature is at least 10C lower than calculated. The difference is even larger if you take the original MCS estimate with the higher room temperature assumptions.