Hmmm. yeah i see what you mean. AFAIK there are only a genuine commodore CD32 pad and the competition pro ones that look *exactly* like the one you see on lik-sang available for the amiga CD32. I dont *think* that there are any other types at all (obviously as you found a megadrive one will work but the buttons arent all there). The one on amigagames.co.uk looks like neither (looks like it might be a snes one? dunno) so I suppose they got you by the short and curlies but it's not good customer relations!
Also AFAIK amigagames.co.uk is that vincent pike fella isn't it? Not a good person to be dealing with IMHO! (please correct me if I am wrong!)