HEllo Paul,
I don't know if there is any difference between the tow bar for the SRi compared to the standard Zafira, BUT, tow bars have to be type approved for the specific model of car. Sometimes sports versions of cars need differnt tow bars because of different suspension and tyres giving differnt ride heights.
Again I do not know specifically about the Zafira but with some sports versions of cars, the vehicle is not approved for towing and thus it would be illegal to fit or use a towbar. If that is teh case then the permisable towed weight is 0 or not quoted in the vehicles specifications.
So please use Peters suggestion only after ensurng it is designed for your model of car.