if you are that concerned get the installer to address/answer your questions rather than random strangers who have no idea of sys/size/req/preferences or location of equipment...a two bed flat would heat quicker than a three bed semi subject to various factors/preferences.
you should know better,than to ask these questions after all its the plumbers fault the previous rad worked perfectly for the last 10yrs and was only changed because they wanted a different rad with a higher output in a different position.......
boiler has reached set temp internally and needs to loose some heat before refiring turning it off does not help the heat,sounds like it is a circulation/dirt issue and external issue,turn it on and leave it run for a couple of hours on 3 or 4 on the stat/dial.
edit. make sure all valves are...