Hello Andrew,
The maximum permitted tow load for any vehicle is set by the vehicle manufacture. It is not a simple calculations based on a percentage of the cars kerb weight, but a far more complex calculation that takes a range of characteristics into account.
Vauxhall have assed the Zafira Mk1 diesel and conclude that it is only permissible to tow 1050kg with this particular model.
The reasons for such a low figure are not published, but it is suggested that the 2.0L Dti engine in this model has cooling issues when worked hard, and thus to prevent such problems the maximum towed weight should not exceed 1050Kg.
Other models from the same range do not suffer this problem and thus are rated to a higher load.