Don't know about the Autohaven, but I recently bought an unused Caranex out of my local free papers and intend to get it converted by the Caranex people to fit the Defender. This costs about £80, but as you can get the car version a lot cheaper second hand, it makes it an economical proposition.
The tent is always the same, only the bit that fits over the vehicles is different, so as long as you remember how much it's going to cost to convert, you can buy any model and have it converted. I'm going to use mine over the bonnet this week, as they couldn't convert it in time, but I do know of someone who has one for sale if you are interested. Cheers, John