We stayed in the flat while it was around 0 degrees outside. In those conditions the double bedroom was unfortunately much too cold and drafty to sleep in.
There is a boiler installed in the double bedroom. To install this they removed the lath and plaster from part of the bedroom wall and bored a hole through the outside masonry which isn't sealed and you can feel cold air from the outside in the room.
The gap and exposed wall was big enough to freeze the room regardless of a new boiler, nest system and radiators on full.
I hope the owners fix this issue as the flat is otherwise as described.