It is surprisingly hard to find specific dimensions of screws online.
I actually think (given I did this myself too) going to shop and manually comparing/trying is best/only option. 
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Dell tech support took a pretty good guess and http://accu.co.uk sell screws for like 1p/screw, so I've ordered a variety.
Thanks. Twitter will use this to make your timeline better. UndoUndo
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This may or may not work, but I'm prepared to go to heroic lengths to not actually have to talk to people. ;-)
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screwed