I have had this several years. I seem to recall having to take a lump hammer to it to get it to "stretch" over the legshields (as you do ). Having retrieved it from the loft, it's still a poor fit on a Series 3.
My question is, is it possible that it's actually for a Series 2, ie. the hole spacings are the same distance apart on a Series 3 as on a Series 2?