Thank you, that confirms my thought that is must be the outside. Yes Graham there is a shoulder but it does not seem deep enough to prevent the disk falling out while fitting the cover and then the wheel on the car. I will try again tomorrow, now I know that I am fitting it on the correct side maybe if I can get the cover on without it falling out I can maybe keep it in place by inserting the bolt through it while placing the wheel in position.
At least the pain was worth it as the spare had only 11psi so needed the top up.
