I'm not sure when they made the change, but my 2005 model has hidden zips in the rear seats to allow access to the ISOfix points without cutting. On the leather uphosltery, there's what looks like a double seam at the back - when you prize the two seams apart, you can see the zip and open it up. I would imagine that the fabric upholstery is similar. I'd have expected the dealer to have told you about these zips if they're present on your car, but having said that, it wouldn't be the first time I've encountered a car dealer who didn't know their product.
Pinched from another website: "No part of the seat belt buckle must rest on the frame of the car seat. If it does this is known as “buckle crunching” and the seat belt buckle could break on impact. The seat belt webbing must secure the seat and take the impact pressure."
So it looks like you need to find a seat with a narrower base that doesn't make contact with the buckle. Failing that, you may have to make do with disabling the passenger airbag and putting the child seat in the front, or biting the bullet and hacking up the rear seat base
We ended up buying a Jane Indy Plus ISOfix and were planning to use it in both the Laguna and the Clio, but it's a nightmare to swap over (after the sales assistant reassured us that it wouldn't be) so we ended up buying another seat for the Clio. To be honest, I probably wouldn't have bought the Jane seat if I'd seen it first, but I went for it mail order thinking I was getting a bargain (an ISOfix seat for 100 quid is pretty good - most cost nearly double that.) My only reservation is that it uses the car seat belt to restrain the child, even though it's intended for 9 months and upwards. The little 'un seems pretty well pegged down by it and the belt goes over his shoulder in the right place, but I'd be happier with a built-in five point harness at his age.