Anyone tried to remove the rear seat backrest from the Estate - just trying to see if it gives me more space for an upright mtb in the back.

You have to fold the seat down abit to take it out

If it's like the hatch there's a latch in the middle a bit like you get on a gate. You just push it forward (or backwards depending where you're doing it from) and it releases the hinge pin from the centre of the seats. Lift it and then wiggle it out from the outside mount.