Well, Alfa came through early for once and my new Zender gear knob arrived after only one week!
(Shame the other stuff that I ordered and was "in stock" is now miraculously on back order
Anyway, it is now fitted nicely and I thought I'd post my experience for the benefit of other would be knob changers who are nervous about taking a hack saw to their beloved vehicle.
Well, the majority of the OEM knob is made up of this carbony type material that is very easily cut through with a hack saw, but sheds everywhere over the interior, so I'd suggest some sheets or something unless you want a thorough vacuuming job afterwards. (like me)
Like most people, I kept the bottomost bit of the existing knob, as that is the lip that the gaiter sits on, and the Zender one doesn't have such a lip.
After cutting the bottom bit off, I found the best way was to cut upwards from the bottom following the steel shaft with the saw, and then cutting in at intervals and ripping a lump off. After getting the bottom half off, it was pliable enough to rip off completely. This leaves you with the steel shaft which has several "notches" on it, and a hard black plastic doobrie which sits around the top inch of the shaft.
At first I tried to saw this doobrie off too using a similar method to the carbon. This prooved difficult though, due to the hardness of the plastic, along with the notches on the shaft making it difficult to follow the shaft.
After getting the new knob and measuring it in situ though, I realised I could simply slice straight through the shaft directly under the plastic bit, and the new knob would sit in around the same position as the old one (The hole for the shaft doesn't reach the top of the knob).
That done it was simplicity itself to fit the new knob and tighten via a grub screw that comes with the new one.
I then tried to glue the carbon rim that I had kept in place just below the new knob using wood glue as someone else has mentioned here. This failed miserably though, as the glue wouldn't bind to the steel shaft.
However, this doesn't appear to be a problem as my gaiter naturally sits just around the base of the new knob anyway, so I left it at that.
Very chuffed with how it looks now. Wasn't at all sure to be honest when I first saw the new one in the flesh, but now it's in situ I'm well pleased with how it looks. Yes, it does look more chromey than the brushed metal look of the center console, but it still goes really well.
I'm unsure whether the chrome handbrake would look as nice, so I have left that for now (Well, that and the fact that the gear knob cost a crotch numbing £75 from Alfa!)
Anyway, hope this helps...