Front left hand corner is where the fuel burning heater is I believe - dunno if they were available in 2001 or if you have 1.
You can disable it by pulling fuse 5 in the engine bay fuse box.
Well as others have said the sensor is proberly foobarThanks for that. I don't want to disable it, just curious as to what it was! I do, indeed have one. After the various replies on here, I went out and sure enough, there's an exhaust pipe for it just ahead of the left hand front wheel. I've just been out in it now and it didn't come on, but it did this morning - which surprised me as it was probably 10 degrees or so this morning. Now it's about 12-14 degrees.