This person should have allowed for this thermostat in the original estimate/Quote. There are no excuses for him to push this back on you as an extra.
He is required to install this new boiler to comply with, manufacturer instructions and building regulations. The latter he has bombed out on.
If he don't play fair, you can complain to Gas Safe Register, but they only cover gas safety, alternatively Trading standards or your local building control office. In any event he has to produce a Part L building regulations certificate for compliance which he can do through Gas Safe Register (GSR) if his registered. Put loads of pressure on GSR if they don't want to know, because if they run a part L competent person scheme for part L they should have power to get the installer to put the work right.
He is required to install this new boiler to comply with, manufacturer instructions and building regulations. The latter he has bombed out on.
If he don't play fair, you can complain to Gas Safe Register, but they only cover gas safety, alternatively Trading standards or your local building control office. In any event he has to produce a Part L building regulations certificate for compliance which he can do through Gas Safe Register (GSR) if his registered. Put loads of pressure on GSR if they don't want to know, because if they run a part L competent person scheme for part L they should have power to get the installer to put the work right.