I used the The Leather Conservation Centre at Northampton Uni and they did an excellent job on the seats of the XK. I read about them in a magazine article but I've not recollection of which one.
I had my seats and doorcards restored at Leather Restoration in Newbury (which I appreciate is a bit of a hike from Sevenoaks). Very happy indeed, very good service, kept informed throughout the process etc.