Can I fit a seat in the rear load space of a vauxhall astra estate/station wagon?

I have a Vauxhall Astra estate/station wagon, I wish to carry 6 people in it. I was wondering if it would be possible to fit a 6th seat the the rear lode area of the car.If I can, will a lap belt be a legal saftey restraint, or will i need an inertia seat belt?|||A lap belt would be adequate, but a normal MOT centre won't be able to pass the seat as safe, you would have to get a SVA (proper vehicle inspection like kit cars have to pass) as it's a major modification with serious safety concerns.