Well, the repair is done - not well, but done nevertheless! The crack looks exactly the same now, except for a couple of little marks at either end.

I'm not going to look at the heating elements now, I'll be too depressed if they don't work. I'll take a look when the weather turns a bit cooler - I can always get the screen replaced then if necessary.

Thanks for all your help and advice! No doubt I'll have more questions for you soon.

Update: I had the "repair" repaired today by a contact who saw my thread about the glasswork on Pistonheads. Top job - if anybody needs a recommendation for a glass repairer in the south-east, just drop me a line.