How sure are you they know what they are doing?We chose a company based on local reputation and in house knowledge and experience. The guy who turned up says he does them all and says all work done in house (they have in house electricians).
Having watched lots of YouTube videos by Urban Plumber and Heat Geeks, I don’t think many heat engineers in this country have the knowledge to maximise the efficiency of these systems.
Have they done full heat loss calculations?