The set belt stop has been lost so the seat belt retracts too far when the seat is laid flat. When laid flat the release mechanism lock as it should when not level. So the seat cannot be raised up as the belt is too tight and locked.
1) Push the two little clips to release and remove the headrests.
Pull the grey lever on the right of the seat base, lift seat base up and forward toward the front seat..
2) See two red capped leversat the back of the two seat hinges and lever them to the upward position, this releases the seat.
Rock the very heavy seat unit upwards and lift complete unit upwards.
3) Take it out of the car and turn 180% so that the back of the seat and base are in the upward position and the seat belt will pull out as it should
4) Replace the missing belt locking stop (I used a tie pin permanently fixed using super glue).
5) Fold seat back down and test then either re-fit seat unit or leave out to decrease fuel consumption..