But now I've stopped laughing, fixed mine in a jiffy with an even tidier adjustment - just undo fixing screw about 1 turn, which allows washer to make full contact with magnet all way around(rather than being fixed firmly at a slight angle as standard) Now stays shut, no problem.
I had the old (and very cheap) arrangement of a push-push catch. I've taken that out and put in two small magnetic catches (B+Q best!) - it works so well, I have to use a screwdriver to lever the fecking thing open now!