wrote

Hi, hope anyone can help before I take the plunge and buy/arrange
plumber!

I have a largish room (14 x 13ft, high ceilings) with a single 700mm
x1400mm Stelrad double convector...

snip..................

Fitting one larger rad in a cold room may well achieve the desired
temperature quicker - but you will still suffer thermal gradients
across/around the room.
For greater comfort I would consider adding a radiator as far from the
existing as possible and also located to warm "drafts".
If you have suspended wooden floors, this is achievable without too much
hassle - concrete floors like ours is another ball game.

Phil