I had the Maxi-Cosi car seat that is part of the Quinny Buzz travel system of 2008. It fitted into the back seats of the Gen6 but the seat belt was at maximum extent so the head was raised a bit more than in my wife's Rover. The base is narrow so fitted in the seats. This wasn't the ISO model.
For when the child is about 1 onwards I have this one in the back of my car:
Recaro Young Sport mircofibre - Black / Aquavit
http://www.prams-pushchairs.co.uk/recaro-y...avit-p-628.html
There is a range of colours available for this model. It fits very well as the base is narrow so is snug in the gen6 high sided rear seats. The seat belt holds it in place, it's not an ISO. Halfords do this range also, not sure who's doing the best price currently.
My only niggle is that the Recaro seat doesn't recline as much as my wife's Maxi-Cosi. I tested the fit of that one in my car and it was such a wide boy. One side was always higher than the other so no good. With the Recaro fully reclined, my 2 year old girl can just about touch the back of the passenger seat when in the fully forward position.
It's a lovely looking seat, soft touch, comfortable for little one. She can quite easily fall asleep in it It should last a long time as it's up to 11 years.
Hope the info helps now and later on. Feel free to ask any questions