It was my mates that first 'discovered' it and the first picture in this thread is my picture taken somtime around 2007.
At some point around 2011 'a guy' got permission to remove it from the tunnels, partially straightened it out for no apparent reason and then tried of flog it at auction for some insanely stupid amount of money. In the process he stole some of mine and my friends photos of it and we started persuing him for copyright theft as the photos ended up in 100s of media outlets as publicity for the auction. Unsuprisingly the car never sold at the auction and ended up at the museum insted. (I'm hoping they didnt have to pay for it but sadly I guess they probably did.) To cut a long story short I never managed to get the details of the guy. The media outlets simply blamed the auctioneers and the auctioneers blamed the guy but wouldnt give me his details.. Hopefully he didnt profit too much out of it as he was clearly just on the make rather than cared about saving it.. I mean why would you ruin it trying the straighten it out!
As for the story behind it is somewhat plausible but it had only done about 11miles iirc so it was hardly even off the end of the line. The tunnels were once full of simlar cars all probably with much more interesting stories but after a fire down there most of the tunnels were cleared out. It was far from the most interesting thing we found on that site but seems to be what everyone remembers becuase of all the publicity hunting for the auction sale.
I'm afraid that your mates didn't discover it as we all knew it was there, those of us that worked there. I myself went down those tunnels under Trentham and the paintshop many times during the 80s.
I've sat in that Mini while it was down there on more than one occasion. There was also a much older Austin 1100 shell near it in those days.