The manufacturer's rated towing limit is not legally enforceable. It is usually based on the vehicle's ability to start on a particular gradient whilst towing. However if you are towing with a vehicle registered from Aug 98, it is illegal to exceed the Gross Train Weight (GTW). This figure is included on the vehicles VIN plate. It is the car's weight fully laden (Gross Vehicle Weight) + the weight of a loaded trailer. So for your Zafira the GVW will be its kerbweight + whatever to arrive at the figure on the VIN plate. So long as that + the Ranger MTPLM of 1179kg does not exceed the GTW, you are legal. However, it is recommended that the MTPLM of the van does not exceed 85% of the car's kerbweight. What you do risk, however, if you exceed the manufacter's towing limit is damaging the car (transmission, engine etc) - whatever caused the manufacturer to set that towing limit in the first place. Somewhat longwinded, but I hope this clears the matter up.
Mike
ps the Ranger 470/4 is a great van - I have one - but be aware the side bunks are quite short, so try before buy.