Originally Posted by Horatio
Pure speculation, but maybe they've used a different auto box in the diesel, and maybe (not unlikely, knowing Renault's track record in this area) it's not up to the job of shifting more than the car's weight plus 700Kg.
But a lesser known trait of Renault is to make a complete dog's dinner of permissible towing weights. What they've probably done is worked out the gross train weight, subtracted the maximum fully laden weight of the car, and given the remainder as the maximum allowed trailer weight. If you write to Renault Customer Services, you may be able to get a revised set of limits for your car, which may allow you to go for a heavier trailer if you keep the car's payload to a minimum.
Yep - getting it from the horse's mouth at Renault Customer Service sounds like a sensible idea...