To tell the truth Dave, I dont think you have a leg to stand on. The spare supplied is fit for purpose in that it is designed to get you home or to a repair place, and that is where Nissans obligation ends.
Any extras you have put on such as a tow bar means that it is then upto you to make sure your vehicle is fit to do the "extra" you ask of it.
Having said that, I cannot stand companies that advertise there vehicles as work horses, but then do not supply the kit for this. Landrover supplies a spare that is identical to the alloys, all be it on a steel rim, so if you do get a puncture with a trailer, then you are fine. Then again, Landys are sold as tow vehicles I guess.