A bit of googling found these two phone numbers for Black Shed Slimbridge.
01453 899 273 and 01453 899 190.
01453 899 190 was the number from many years ago, what with the fast turnover of telephone numbers when they become available and the length of time the black shed has been closed, I'd be amazed if it's still valid.
I suppose the trick is to try them during reasonable work hours.