Hello Sandra,
I havn't used a Zafira for towing so I cant make any specific claims for or against it, But I do wish to correct a point in your post.
85% is not a limit, it has no legal authority. The towable limit of any car is deteremined by the car manufacture and is stated in thier specifcation. For some cars it may less than 85% and in many cases its is actually over 100%. For some cars its actually zero and they cannot tow.
Drivers who passed thier driving test after Jan 1997 have greater restrictions on what they can drive and tow, which effectively limit the caravans mtplm to no more than the unladen weight of the car.
With regards to the 85% figure, it has no magical properties, it does not gurantee a safe tow, and outfits can still become unstable or difficult to drive. Equally exceeding the figure by a few percent does not guarantee and outfit will be unstable.