Renault Laguna Catalytic Convertor

I have a Renault Laguna 2.2 dci Estate Initiale on an '04 plate. The vehicle has an extended warranty by Renault For You - this cost £1200. It is for 2 years or 100,000 miles. The vehicle has currently done 66,000 miles and has been fully serviced by Renault. The catalytic converter has failed and needs replacing. This item is covered under the warranty so I took it into my local Renault dealer who examined it and costed the job so that they could put the claim to the insurance company. The cost was in the region of £1400. When I questioned this I was told that Renault had modified the cat and the extra fittings to take the modified cat would cost approx £450. The insurance company agreed to pay for the cat but will not pay for the modified fittings. The cat cannot be fitted without them. I am now being asked to pay for these bits! It has been suggested that a non-genuine cat is fitted but if this failed it would not be covered on the warranty. Has anyone experienced this problem? Any comments?