[PaulR]

I cut pieces of corrugated plastic sheet (the type you get pinned to the garden wall advertising the builder when you have had new windows or similar) to shape. These just slot in nicely behind the radiator grille to form an invisible radiator muff, admittedly only covering half the radiator. I used it during the very cold winters we had a few years ago and felt that it did help to keep the engine warm when the heater was running, but it might have been wishful thinking!