Two solutions, one slightly harder to carry out than the other:
A) Using an angle grinder and some nice brass door hinges from B&Q, cut a rear door out of the rear wing - sorted, no more I-can't-get-into-the-back-misery....
or
B) Unpick the seat cover and thread a new cable. My drivers seat is broken too and a mate reckons it's not that hard, although I can't be arsed and simply reach under the seat and flick the catch by hand if anyone is that desperate to get into the back.
