I've browsed this because I also have a newbuild house in the UK and am underwhelmed by the heating system and the quality of work in the installation.
One problem I had is that the downstairs thermostat is in the hall which is quite a small, enclosed space, but the kitchen is large and has a flat roof. So the hall comes up to temp, the thermostat switches off and the kitchen is still freezing. I then put in a Hive system and the thermostat is now removable from the wall, so I put it in the kitchen. This also meant that I then needed a TRV on the rad in the hall. But I still have one rad that consistently refuses to come on and know the problem is not the TRV.
One very specific tip I would give - the developer has probably given you a two year warranty on the house including appliances, before the NHBC clicks in. Here is the BUT...the heating system including boiler, hot water tank etc. needs an annual service by a heating engineer approved by the manufacturer. If you do not do this service, the developer will wriggle out of the warranty on the whole system in the second year.
Secondly I'd back up other comments, don't try to fix everything yourself, the developer has given a warranty and must fulfil that. Do not be afraid to call them back again and again, especially (but not only) if they are still on site, because at the end of 2 years they will walk away. Then you really will have to sort it out yourself. Of course the big problem is that the developer has subbed everything out, including the warranty repairs.