Could this be a similar problem which effected the Citreon Xantia in extreme circumstances?
The Xantias handbrake was applied to the front disc brakes and when the discs cooled they shrank back to their normal size hence creating less grip when the handbrake was originally applied when the discs were very hot and expanded.
As the MINI has discs all round this could be a similar problem???
Your garage might not pick up on this if they tested the brakes under "cold" conditions.
What you may need to do is recreate the conditions.
Just a thought, I hope it helps
