I've also e-mail Vid Roads and asked them specifically if there are any particular laws regarding age/height for the 3rd row etc.
Better to be safe than sorry... we really only want to for when we are going somewhere with a few people and need the extra space in the front row (option to move the middle row forward to allow another adult in the back too).
I wouldn't mind getting one that would suit DD1 right through to my nephew so he can fit in the back seat too.