It is a requirement of current building regs to fit a room stat.to conform to efficient use of the boiler and be a good carbon footprint user. Currently you have no direct control of the temperature in perhaps the room you use most at present the boiler is the desired or set point of control not the room where the stat should be situated,however many homes are like yours and only come under scrutany when a HIPS type survey is carried out. Your original installer has but a jump wire between terminals 4-5 on the wiring terminals.If you do fit a room stat it would need to be supplying 240v to these terminals,newer models are logic driven and have to be voltage free devices. Regarding the temperature control your boiler has a CH knob which when moved displays the required temperature for 5 seconds the display reverts to actual returning water temperature, rangeable 20c-90c. you say"it reaches maximum before it goes off" surely not at 90c. I think with respect that you read ,digest and understand the manual in greater detail to get more efficiency from your boiler also if there is a programmer involved make more use of it it could also save you money.
Guytawe, May 2010