If you look at Woolies website, you need 10 metres of 50mm wide Pirelli webbing, 4 x webbing clips and 8 x webbing plates:
https://www.woolies-trim.co.uk/produ...irelli-webbing
If you go ahead and do the front seat back rests, then you need some 38mm webbing for the "up-and-down" strapping, for want of a better description. Originally, the lumbar support control webbing would have had 50mm Pirelli webbing (the side to side strapping which is controlled by the lumbar support control knob). However, I have found that fabric webbing or old seatbelts work much, much better.
It's a time consuming job, but very rewarding resulting in superbly comfortable and supportive seats.
What year is your car though? Some of the later ones had a metal spring base underneath the seat squab.