Ah, so full of hope, so nieve, things I have learn't...

Never remove the seat cusion from a seat, unless you are prepared to take the fabric off the seat - it is difficult to get out without damaging.

It is really difficult to get out, and putting it back in without damaging it is even harder.

Easiest option is to either get the seat sorted out professionally, replace using a non-standard seat, or wait for a set to come up for sale and put the "new" passenger seat in the drivers position.
my seat is fine now,

it is a little out of shape - because I packed the base up with plywood since the bottom was dropping out, I have also lowered the whole seat by removing the aliminium spacers between the runners and the car, (to negate the fact that that I now sit about 2 inches higher in the car, and the windscreen top bar gets in the way) .

On the positive side, it is comfortable for long journeys

- no dead leg, which was the point, so it doesn't look the best - not quite buckety.