Finally got there. Having tried to remove the pedal box and failed due to the number of other things that get in the way, I removed the fusebox, loom connector panel and relay board, plus the lower dash panels. Then I used a 90.deg. drill adapter on a battery drill to chop out the side of the plastic box surrounding the electrics, by poking the drill through the fusebox hole. There is already a rectangular cutout that I continued to the top of the box. I could now put my hand on the quadrant and loose nut. When I'd got it rotated to the best position, I pushed up on the quadrant using a long screwdriver alongside the throttle pedal and tightened the nut. I also replaced the nut with a locking item. You need to get rid of the washer to have enough thread to bite into the locking part of the nut. An alternative is to replace the plain washer with a nordlock. Getting to this position took a month, putting it all back together took under 20 mins. Starting from scratch, it should be under a couple of hours. The connectors are all colour coded and the relays only go one way.
What I have realised is that there is no endstop on my throttle pedal so it runs out of travel before it's hit the carpet. I think this is why the quadrant came loose. The part no. for the missing bit is 6N2 723 527.