Hello Graham,
Sadly the Mk1 2.0Dti Zafira does have a dismally low maximum tow weight, for the engine and vehicle size, and whilst the unladen weight of your caravan may be only 975kg, the regulations do not use that figure but the MPTLM of the caravan which must be no greater than the maximum permitted under the type approval figure of 1050Kg.
The calculation of the maximum permitted towed mass for any car is made by the manufacture and is model specific. It is based on a combination of tests, and engine-cooling capacity is only one of the many factors considered. The fact that the same engine is used in a different model and is allowed a greater towing load cannot be used to ignore the manufactures limit.
Other engine variants of the Zafira do have a greater towing capacity, as does the MK2 Zafira diesel.