"Recommended" save towing weight is 85% of the towing vehicle weight. I've towed the Cossie on a trailer with the Mondeo estate and it didn't feel happy at all, used the Smax a few time s now and it felt a lot better (both cars 2.0 diesel). A lot of it depends on how you load the trailer.

As said, if you get pulled doing this and you're illegal you can get in a lot of bother.