The genuine BERU radiator seems to suffer on many of these after 10 years or so, they are available from NISSENS and best buy is probably on ebay (mine was).
Exchanging can be done without dismantling or disconnecting any AC system component if you exchange it from the top rather than drop the complete package as described by Renault.
You can see some picture from when i replaced it here:
http://www.servicehefte.net/servdata/?pid=czqkk-qqqXShould give you an idea, looks worse than it is. Quite simple actually, just a lot of dismantling. 1-2 hrs and you are done if you have all tools etc on your hand.