Had an email from the dealer earlier, new wheel with balanced tyre will be available fro 14:00 tomorrow

Dealer advised me there is no distance limit on the space saver but only has 3mm of tread so limited in that sense, also incredibly noisy, looks awful and have to drive like Miss Daisy! But will get me through a few days of commuting.
I do think this sort of damage would have happened regardless of manufacture, would make an interesting test for the Mythbuster brothers! Bit like the frozen chicken test on cockpit windows.
As i was driving in a straight line I think the front wheel must have run over it first and kicked up the offending lump which hit the rear tyre awkwardly, something had to give at that speed.
First time in 24 years anything like this has happened....we all need to keep a good look out for debris and pot holes, safe driving everyone