It’s being fitted because in all probability you have a water meter or check valve on the water supply, it’s giving the expansion of the hot water somewhere to go, it should be fitted if the company want to install it.
I would not worry too much . Nobody is going to ask for your registration papers, as long as your plumber has done a good job enjoy the benefits of a pressurised water system.
Infrared thermometers are fairly accurate as long as you understand that not all materials are suitable, shiny copper for example will give wildy misleading results, google emissivity.
Most likely the boiler is full of air, there should be a air vent inside the boiler but you should not take the front off the boiler , what about getting the plumber back that done the drain down ?
I would like to see you walk into tescos and argue about their prices at the checkout.... perhaps you could make your own bread and then complain that they are making nearly a billion pounds in profit....
Although its a different model to yours its the same manufacturer, you will probably find there is enough common ground to allow you to programme your timer.
Einleitung (altestore.com)