just an update I managed to fix this,
Theres 2 hooks at the top of the seat pad (on the main seat base). The pad is held onto them via a metal pin the length of the pad. You basically have to pull this out to release the pad, then theres some spring clips lower down which can easily be seen and unclipped.
The issue was that the roller is connected to a cage via a tensioning wire you tension the wire via the roller and it tensions/pushes out the cage giving you lumbar support . The metal hook between the wire and the cage had snapped. I used some metal rope (I experimented with some fairly heavy gauge electrical wire 1st, but it snapped when i out weight on the pad) to replace the broken metal clip.
Replace the pad and put the metal pin back through the hooks and bobs your uncle!