Managed to get round this. In case it helps anyone else... I hacksawed off the broken plastic end so I could at least remove the old assembly. I then had enough room to get some molegrips in behind and grab the loose thing while undoing the nut.
Job done now. Interestingly, the part I replaced looks identical to the new one, whereas I thought this had been re-engineered to fix a design weakness? (mine's an '02 147 so I assume the part I took out was an original faulty one, though it has lasted 11 years so can't really complain I guess).