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Right it looks as if the teeth are NOT knitting together properly. I've compared it with another zip on the same thing and that zip is tight, but I can see little chinks of light through this zip.
I've managed to get it closed for now. I can't see where the break is because of a sleeve at the base (top) of the zip, so when it is not quite so windy I'll go and unpick the sleeve to see what I can do.
I should have just bought a bloody greenhouse - trouble is, with the tree where it is, I can't actually fit the size greenhouse I need in there.