Wednesday, I drove my MINI to school in the morning, to work in the afternoon, and home in the evening. It sat in my garage for about 20 minutes while I prepared for Bible study that evening. Up until then, all systems were normal. After that 20 minutes, I got in the car and started it up...holy crap. The first thing I noticed was the temperature gauge, which was all the way up in the red zone, my tachometer was at 0, although I could plainly hear and feel the engine running. the outside temperature display was totally off, the tire pressure light was on, the DSC light was on, and the speedometer didn't work, either. I drove it around the neighborhood just to see what's what, and every once in a while, everything would flash for a second or two to its normal state, then go back to the "problem state." I unplugged the battery for a while to reset the whole system, and when I plugged it back in, everything was perfectly normal for about 15 seconds...then it went haywire again. I feel like something has just come unplugged or loose, if someone could tell me just what that might be, I would greatly appreciate it!!
After looking into it, it looks like it is in fact the battery. There goes more money that I don't have...
On second thought, it's not that. After buying a new battery, everything is still acting up. Alternator is putting out plenty of voltage, so it's probably not that, either.
Since you have "There's nothing like the sound of a screaming R53 =D" I assuming this is a 1st gen MINI. Pop the hood, look for the long rectangular relay box on the right side (as you face the car) and open it up. Press down on all the relays one at a time to make sure they are securely in place.
Also on a 1st gen car...mounted on the driver's side of the firewall is a small plactic box that houses the positive jump stud. Check that stud for corrosion and tightness. I had a similar issue with the charging system on my first MINI and it ended up being the jump stud coming loose.
But the money spent on the battery wasn't wasted. Those should be replaced when they fail or roughly every 5 years. From the THMMC site I saw you have a 2003, so that one was probably due soon.
It could be the plug at the tach, take the screws out and pull the plug and reinsert it firmly to make sure - it's a no charge check. Other than that, I'd look for a bad ground somewhere too - make sure the battery cable has a good connection to the car, and the one in the engine compartment too - that one's up near the motor mount on the passenger side of the car.