I'm no expert but if the seat was fitted at some point, chances are it can be removed, it may just need the experts.
Stick one in the back of the car and take it round to your local upholsterer for them to have a look.
Thanks for your replies. I decided to google and Plumbs the well known firm, say on their website that they can re-upholster any furniture so I shall get in touch with them for a quote.