Doing some winter maintenance knelt on the seat and thus split the 21year old membrane, I had also previously broken the pulling part of the tonneau cover zip.
Took the tonneau to B J Car Trimmers, I had used them many years ago to make a hood for an Austin 12 tourer: Barry repaired the zip at no charge, and the following day I took the seat to him for a new membrane, this was ready for me to collect the next day total cost £ 30.00.
His workshop is in East Hanningfield. Essex, I can only recommend him highly to anyone needing trimming work.
Usual disclaimers