Designing a homes heating system is likely the hardest thing to design in a modern house, the first point is of course the design of the house. I have lived in three homes, each one very different, two were controlled by thermostats, one was hot air so it did not matter where the thermostat was air circulated, the next was open plan, so again air circulated so again thermostat worked well, the last was a traditional house with doors on each room, and no vents in the doors, and thermostat radiator valves (TRV) in each room, they were also fitted in the open plan house to stop bedrooms over heating, but in the house with rooms and doors on every room each room is controlled independently.
The plumbing system is designed for this, as each TRV closes then the return water gets warmer and warmer, likely lifting a by-pass valve and the boiler flame height reduces giving a seamless hysteresis free control. As it reaches the point where the boiler can't reduce flame height any more it starts to cycle, built in anti-cycle software reduces how often the boiler fires up to test the system, however it can't turn the boiler off.
So now comes the hard bit, some how you need to turn off the boiler in the summer, so the options.
1) Manual it's warm enough so you switch it all off.
2) Fully automatic the TRV heads are replaced with electronic heads which in turn connect to a hub, when every head reports satisfied then the hub turns the boiler off.
3) Some thing between the two, common idea is fit a thermostat in the coldest downstairs room, which also has no alternative heating, and no doors to the outside and don't fit a TRV to that room or turn it wide open.
The problem with 3) is often there is no room to fit the bill.
So one can understand why neither the electrician or the plumber wants to get involved, the plumber is simply a worker of lead, what plumbers do today not sure, as very few lead roofs or lead pipes, the pipe fitter is very skilled but these people work on 36 inch heavy walled pipes not little pipes up to 6 inch, I would say some one who does not work with lead, but uses plastic or copper is only semi-skilled.
The gas man and the heating and ventilating engineer however have skills the old plumber did not need, but step one in every trade is design.
So who designed the installation, and what did the designed ask the electrician and/or plumber to do? Many of an installation job I did not have a clue what the wires did, plans said run cable from A to B and connect wires to terminals UVWXYZ and that is what I did, some one latter came along and commissioned it.
So in your case the thermostat may go direct to boiler, but it could also go to a motorised valve, and it could also go to a hub. Without knowing the system we can give totally wrong answers.
