In reply to Rick Graham:
We bought a boot jump last year and I can't sing their praises highly enough. Absolutely transformed our experience of using the car and a brilliant, beautifully crafted piece of kit. We must have slept a total of something like 80-90 nights in it over about 15 months and can't really fault it at all, except maybe on price! it isn't cheap, but in our experience worth every penny.
It lifts in and out easily, can be used with all or no back seats in place which of course how much extra storage space there is, and works amazing well as a campervan, with the table and stove etc.
Initially I thought the spirit burners were a waste of money, you couldn't buy without and they are very expensive Swedish make, but again I am a convert. Bascially very large (2x1 litre) trangia's but so 'docile' that I am happy cooking inside and outside the car.
Before we got the Boot Jump, our experiences of camping in the back were mixed. Basically fine in good weather, though a hassle shifting stuff around to create sleeping space, but a pain in wet weather.
Contact me if you want more info. I am no way way connected with them, just a very satisfied customer.