My '06 has about 76K on it, and I've not been driving it much lately. It is stored indoors. Today I swapped out my winter tires for summers (yay!) and took it for a celebratory spin. My last stopsign before home, the brake pedal went nearly to the floor and I started looking around for an anchor to heave out. Fortunately it did stop but something was definitely wrong. Interestingly, the next couple of applications were more normal-feeling, though maybe not 100%.
I checked the reservoir - it is full. Fluid is due for changing but I don't think that would be the issue here. I will be getting back under the car to check for leaks, but I'm doubtful that is the issue.

I suspect the master cylinder, but wanted ask whether there are Mini-specific quirks I should know about when investigating. I thought it was odd that the pedal was suddenly soft (after a mild drive) and then almost normal again. Does that sound like a MC to you?

Any thoughts much appreciated!