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The RAC have looked at it and photographed the wheel and damage. The spacer for the tyre was put on the wrong way round and hammered into place. Wheel nuts too short so the thread was stripped. I won't bore you with it all, but suddenly they can't remember who changed the tyre.
Luckily my son was only doing 5mph as he was parking at the time, but the wheel flew off and could have hurt someone--the RAC guy said that if he was going faster, the car would have flipped.