Go to a Renault dealer with all your documents (v5 for VIN) and ask them if it is covered under the Additional Warranty that Renault have issued for Lagunas with Turbo Problems. It's such a common fault you can get your turbo replaced and all your labour (at a renault dealer only) paid for if it one of the cars that are a problem. Some friends of ours got their turbo paid for but not their labour as they had it done at a different garage before they knew about renault paying for such a repair.
It's worth a try.