Thanks Bubba. I finally got the cylinder out. I had read about the "C" clip, but didn't realise how big it was or that other things were incorporated into it so I kept dismissing it!
The link you sent me is great - I've used it in the past, but www.bmwe30.net
(where a lot of the tech help is stored) has been down for the past few days at least. Hopefully they're not down for good.
Since I had such a hard time with this I'll post some pictures a little later on when I get the locks switched around