The rear seat conversion on my plus 8 was completed prior to my purchase of it in 1998, I believe it was carried out by Libra Motive in Colindale, London.

REAR FLOOR AND SEAT BASES
 

It is quite simple. The carpeted rear floor panel (over  the battery) has been replaced with a similar panel that contains two small,  sunken, padded seat bases. Separate to the bases are two padded seat backs and headrests which are attached to the rear of the cabin to points under the stowed hood frame.

THE COMPLETE SEATS
(minus the safety harnesses)

Attached to the brackets which hold the seat backs are three point safety harnesses. The seat backs can be removed in about five minutes to access the battery.

The panel is carpeted to match the interior and the padded seats and backs are in leather to match the front seats.