Did you add extra insulation to your house (or was it already more modern?).

Otherwise the heatpump just can't catch up.

And currently I have the opposite problem. The house is too well insulated for the heater (or the heater is too powerful). The heater only runs for a couple of minutes and huts off.

Hah, no my house was built in the 1750s or something like that - single glazing on the front and with fairly minimal insulation, plus it has no cavity wall (a mixture of wattle and daub and brick). It is a mid-terrace though so I benefit from neighbours being either side.

There have been a few cold snaps here where the weather has been down to -2 some days, but it’s been fine. I had a couple of minor installation issues (eg 3 way valve set incorrectly) but once those were fixed my house hasn’t dropped below 19C.