So if from tip to tip it is 80mm, then you will have 70mm from screw hole to screw hole. A little geometry will let you figure it out :D".
So I ordered a 80mm fan, which turns out to be far too big. Just to clarify - the fan I bought measures 85mm diameter from blade tip to blade tip, and the holes are 83mm apart (measured from centre of holes). So far bigger than the 70mm.
I really didn't think this would be too difficult - is the fan I bought (new on TradeMe) not in actual fact a 80mm fan, or do I need a 60mm fan, or (knowing my luck) some kind of mutant variety only available from a car boot in Puerto Rico?