whys my house toooo hot?

Thanx for your reply to my 1st question.
The stat is in the hall, and the system was put in nearly 6yrs ago and was working fine. The problem is, when I turn up the stat to 22, if I then need to turn it down, the rads stay very hot never cooling down as previously did. If I turn off the heating system at the control panel, and leave it on hot water only, the heating system still stays on aswell.Thats why 1 engineer put in a new actuator.