Completely agree with this and other posts suggesting the same/similar. So.....given that you are where you are, spent your money, and presumably don’t want to rip out your brand new ASHP and start again, the first and most practical question is can you do anything about the heat requirements of your house? You don’t mention anything about existing insulation/airtightness in your opening post, but that’s surely got to be the place to start?
First seal any obvious drafts. Second investigate/seal less obvious sources of air leakage (almost certainly lots in a 1970s build...... service pipes/wiring which enter through external walls/floors/ceilings....suspended floors, particularly timber, which have almost certainly shrunk/sagged, and all the points at which they meet the walls. Third, can you improve insulation of the external shell? Look at the ceilings first, floors second, and walls/windows third, depending on what you find. The more you can do to narrow the gap between your heat requirements and the capacity of your ASHP (at sensible cost) the more comfortable you are going to feel.