Do your heat loss calcs based on 40w/m2, [/quote]

If you were doing you heat loss calcs based on 40w/m2 I would be worried...That would exceed new build regulations by 5w/m2, and that is as rare as hens teeth in a retrofit and very rare in a new build...

Sorry to say it but if you do it on 40w/m2 your heat pump will be too small and your running costs would be much higher than they should be.