Radiator lining. Wall-mounted radiators have a design flaw – they radiate heat in all directions, one being directly into the wall. And with the big wide world on the other side of it, there’s no way you’re going to appreciably warm that one up. The result is a huge amount of lost heat energy – up to 70% in some cases.
The answer is to put something in the way. There are various hi-tech solutions, such as this radiator booster, but there’s a quick and cheap way to do it yourself, which is to line the wall with tin foil, the most reflective side facing the radiator. The heat will be bounced back into the room and your heating bill will plummet. I’m sure you can think of an artful, tasteful way to do it.