To get at the assembly I folded the seat up and attached it to the side. By close inspection, I found a zip along one of the seams on the underside of the front of the seat. Pulling out the end of the zipper with my finger, I found I could unzip the seam and then partly push the cover to one side so I could see the sliding mechanism. It uses a cable (like a bike brake) which is linked back to the handle which you use to release the seat to slide. And I also found the cable stop had come out and so there was nothing to push against to release the foot sliding lock. (sorry I didn't take any photos)
I did need a good LED torch and a bit of patience to sort this out. I also had to unscrew a few philips screws to release more of the plastic covers and also used an angled set of pliers to pull off the push fit plugs used instead of screws for a few of the fixings. (The angled pliers mean you can grip the trunk of the plug rather than the top which is flimsy and would tear off). Reassemble in reverse sequence.
So now both back seats slide back and forth as they were meant to!
