It's been inthe regs since 2002, so it must be already included in his quote
For umpteen years, good heating engineers have been fitting roomstats to control heating systems.
Each room has a design temperature. Living room 20 degrees C, hall 18, bedroom 16 and bathroom 25. Heat loss is calculated with these temperatures. Therefore a radiator fitted in the lounge will maintain lounge temperature at 20 when external temperature is at -1. It follows that a roomstat fitted in the hall and set to 18, will switch the boiler off when the hall reaches 18 degrees. By this time, the lounge will be at 20, bathroom at 25 and bedrooms at 16. Same roomstat if fitted in the lounge would be set to 20.
While a roomstat will shut a boiler down (conserving fuel), TRVs will stop taking heated water but fail to tell the boiler that it should stop. Meanwhile, the boiler has now to rely on its own stat to shut down. It keeps firing every so often to maintain the set boiler temperature.
If the TRVs are used alongside a roomstat, rooms that are naturaly warmer (say south facing), shut down by the TRV. Colder rooms continue to heat until the roomstat (hopefully fitted in one of these rooms) switches off the boiler.
If the radiators are oversized, guestimated or not balanced, the roomstat will get erroneous readings and by unable to function correctly.
Invite your engineer to carry out heat loss for the rooms and see if he is able to do that, if not choose someone else..
Modern digital roomstats can control room temerature to plus minus 0.5 degree