We recently visited a modern flat, late 1990’s and we found some tiny draughts round the window and door frames, these seemed to have been sealed with silicone rather than acrylic sealant designed to be painted over. These draughts were all but insignificant but there were draughts emanating from the trickle vents even when closed. These vents were surprisingly insubstantial and it was rather a shame to find that they were letting in draughts. It would be possible to seal them to the frames to prevent some of the draughts but this would be tricky. The windows and doors were very high quality which makes the trickle vents shortcoming all the more surprising.
The front door had smoke seals which were unfortunately not closing the gap between the door and the frame. Light was visible from the lobby outside, this should not be the case and if yours are like this they should be reported and dealt with by replacing with ones that work even if this means adjusting the frame.
The bathroom fan was functional but in our opinion not ideal. Some flats have had problems with excessive condensation leading to mould in their bathrooms. There are now fans available that are much more controllable and nearly think for you. I was personally involved in very successfully installing 36 of these in newly renovated social housing units a couple of years ago. The idea is that the fan runs slowly all the time and turns itself up if the room is used, and even more when it gets humid, sometimes needing to go on boost, returning to trickle mode once the humidity drops. It can be told not to come on at night, all the speeds can be adjusted and the fan is simple, safe and easy to clean. The fans are obtainable on E-bay at reasonable prices, I got all the ones for my project from there. List prices is crazy high but group purchase should be possible but likely higher price than online. https://www.vent-axia.com/range/lo-carbon-svara
We had concerns about the insulation barrier suffering from thermal bypass but it is beyond our remit on DraughtBusters visits. Thermal imaging would show up any problems but a key indicator would be draughts emanating from socket outlets which we hope will not be the case.
DraughtBusters unfortunately do not have the resources to visit individual flats, we offer this information in good faith and without any liability. Further information is available here https://readinguk.org/draughtbusters/