I had this problem after a rebuild by a mechanic I will not name for reasons about to be explained. I'd just got my 1275 back from a rebuild after someone who obviously thought I was a bit of a ning nong (understandable) poured sand down the oil filler cap (not so understandable).
I got the engine home and put it in my car, all ready to go. Primed it, got it running, gave it a quick "by feel" tune and all seemed well. Took off down the street and noticed it started being a bit funny on gear shifts. Then it stalled when I put my foot on the clutch.
Called the old man who was grascious enough to tow me home and pulled the clutch housing off, all seemed to be ok. After a bit of poking around I found that the Thrust Washer (?? The brass "C" washer that holds the primary gear in??) was missing. Presuming that the clutch had been set up wrong and it had fallen into the gearbox I started pulling the engine out.
Took it back to said mechaninc and started pulling it apart only to find no thrust washer in the sump or gearbox, or even an metal filings. Mr Mechanic had not put it in to begin with, and everytime I was putting my foot on the clutch it was pressing the crank about 5mm towards #1 piston.
This, of course, stuffed everything. Crank, bores rings, pistons, all sorts of crap. He rebuilt it and sourced fresh parts for no cost which was nice, but damn, if he had of just put in that bloody washer....
</rant>
So anyway, I hope it's just a bearing as I've gone through my fair share of those and it's no biggy. If it's not,

my thoughts are with you and your Mini
Peace.