ASHP and supplementary heating

Meatballs

Rads - yes same calculation you have used just expressed slightly differently - but yours is for a dT (0.406) of 25 rather than 20 (0.304).

500/2.4 = 208W
500*0.406 = 203W

e.g. 
https://www.stelrad.com/wp-content/uploads/2014/08/stelrad_technical_info.pdf 
https://www.buildingservicesindex.co.uk/entry/136540/AEL-Heating-Solutions-Ltd/How-to-calculate-the-delta-T-for-a-radiator/

Reed_Richards

The actual heat loss calculation assumed slightly different ambient temperatures in different rooms so strictly the conversion divisor (/2.4) should have been different for these different rooms to reflect this. But it wasn't.  My maximum flow temperature is set at 50 C and the return temperature is nominally 45 C so if the room temperature is 21 C that would give me a delta T of 26.5 C.  If I am prepared to pay money to warm up the house quickly I can increase the flow temperature to 55 C which should increase the radiator outputs by something like 25%.