right ive figured it out, its obviously on the cars fron new but its quite awkward to remove and even more so to re fit so obviously whoever first took the spare wheels off most zafiras just never bothers to put it back as its not really needed, having said that it does plug the hole in the floor so if your car doesnt have one then you must have a hole straight through to outside which is approx 40 mm round so could possibly let splash water in and wet your rear carpet.
This bolt would also stop anyone trying to steal your wheel, although a determined thief might be able to snap it off with it being plastic, its m12 size i think though so quite hefty.
You have to lift the rear right seat and theres a hidden plastic bolt head under a little flap in the carpet, you have to hold the wrench on the bolt head inside the car while spinning the wheel under the car to undo it and drop it down, its a right faff.
My spares brand new with nobbles still on so obviously no one has ever gone through this rigmarole before on my car, i am the lucky first ,lol.
Aaaah well that might help someone in future!!!