I've put a piece of thick card betwixt the rad and grille for the cold weather, and it seems to be doing ok. The car warms up nicely, and the heater is up to something like warm within a few miles. It's not a very large piece of cardboard - maybe about 10" the same as Matts, with a piece of string looped through and hung over the radiator cap. It made an entertaining picture for the after-school club as I decamped to the staff carpark armed with string, scissors and an old copier paper box!
As I said, it works well.

There's a slight leak on my radiator which produces some strange effects - now the radiator gets up to temperature, the moisure that collects on the rad is more likely to turn to steam if there's sufficient humidity in the air... the first time I fitted it, I reached home in a Flying Scotsman-like cloud of steam!

Hasn't happened for a while, though, and YES, I have checked the water level... ;) It seems to cope with slow-moving and stationary traffic pretty well, too - as my 45 minute crawl home through the snow yesterday...