Using the formulas here:

http://wiki.diyfaq.org.uk/index.php?...ting_radiators
If your radiator is a double panel single convector (type 21) then its delta 50 heat output is only about 1197 watts. If it is a double panel double convector (type 22 - thicker with two sets of fins in between) then it will be about 1512 watts.

However your average panel temperature is only (66-59)/2+59 = 62.5 degrees which assuming the set point is 20 degrees that falls to about 1068 watts and 1350 watts respectively. IMHO that is considerably undersized (especially the lower figure if its a type 21) in terms of wattage for a room that is 3.9 x 4.3 x 2.4 metres.

By way of comparison our living room is 4.3 x 3.9 x 2.8 metres with a 1 metre deep bay window with brick and plaster interior walls. There are three separate radiators mounted around the bay window piped together (with a single HR92) each 800mm wide by 600mm high - they are old fashioned double panel radiators that do not have fins, so are lower effective wattage than modern convectors would be, however their total delta 50 wattage still works out to about 2800 watts - roughly twice the wattage of your radiator, and visually more than twice the size.

According to my graphs the room takes about 1 hour 50 minutes to go from 11 degrees up to 21 degrees in current condtions where outdoor temperatures are between 0-5 degrees. However keep in mind that I am measuring the temperature at the HR92 (no separate wall stat) which will tend to give the impression that the room warms up faster than it really does (the far side of the room is not up to temperature that quickly) however a separate thermostat in the middle of the room shows that the room is up to temp (21 degrees) in roughly 2 1/2 hours from 11 degrees.

So I would say that yes, you are asking too much of your radiator, even just looking at the picture my immediate thought was "too small", but doing a few calculations confirms it.

You say you don't have much more room but it looks like you could increase the height significantly (bringing it up to just under the window) and if it is only a single convector type 21 you would see a significant increase by going to a full thickness type 22 without increasing the surface area on the wall over a type 21.