Can categorically state that removing the rear seats does not invalidate insurance having spoke to my broker today (works with lots of insurance companies for lots of different commercial firms).
Also from personal experience with land-rovers, seats always in and out. V5 said it was a 9 seater. When I crashed there was 6. (2 front, 2 middle, 2 back).
However, insurance does need to be notified, as always with ANY change, even a change of colour/wheels.
Also, from a personal viewpoint, I wouldn't remove the rear seat, cos then where would I keep my empty macdonalds bags?
EDIT: By the way, didn't post this to stir. Just though a few people would like a definitive answer.