I may have to do this at some stage because I don't think that my Roadster screen demists, even though the dash light comes on. I tried it during the cold weather - breathe all over and switch on. Nothing happened. Mr Bumble has a very patchy heating element. Using the same test there's a large patch from the centre-line of the steering wheel to the offside which doesn't clear at all. The rest is reasonable but not uniform. This was a big problem in Scotland last year in the wet. The demist may be required by law but it's not very good if my experience is anything to go by.
