Update, not knowing if a space saver will fit under the seat, I've bought a good condition steel rim from a seller on an internet auction site and have fitted a brand new non directional budget brand tyre to it. The intention is to keep it in the garage and if we suffer a flat, then at least we can keep the car running without the restrictions that come with a space saver. Total cost, £70. Our intended use of the IQ will always be for local use, so in the worst case we should be able get the tyre out to the car with no issue.