[Larne]

It wasn't so much the concept of a fixed maximum f-stop throughout the range of the lens but a fixed maximum aperture diameter.

Taking the example above, if f/5.6 at 55mm is an aperture diameter of 9.9mm then why when the lens is at 18mm can we only get a max aperture diameter of 5.1mm. If the lens could maintain the 9.9mm maximum aperture diameter throughout its entire range then the lens would do a maximum aperture of f/1.8 at 18mm (18 / 9.9).

That's the bit that I don't understand - as I said above, I can only assume it is in some way dictated by the mechanics of the zoom lens.