...and if you use the brake lights, the car stops - on it's own!

I am not sure whether you have an alternator setup or a dynamo. If you have a dynamo, then what you are describing is the drop in woltage as the dynamo spins down, and the regulator is not switching fast enough. I would first start with a look at your regulator unit to see that it does switch immediately when you take your foot off the accelerator. Also, if you have a wide-belt pulley, the tension needs to be bang on or the belt slips badly on the over-run.

If you have an alternator setup, I'd be inclined to have a look at the state of your battery as usually the regulators are solid state, so that should not be a problem (unless you have a duff alternator or a slipping belt). Mini batteries are usually pretty inadequate if you use the standard spec, so I have always gone for higher Ah ratings and the largest battery that will physically fit into the battery box. Also, the battery earth lead connection to the chassis can corrode quite easily, so have a look there (mine's in the boot).

I have a Mk1 installed in a '91 998cc Racing Green (not as nice as yours - look on the geek site for a picture). The Mk1 is a simple on-off two lead wiring setup, so I wired to body earth on the front bulkhead and tapped with 17A rated wire from the ignition feed. For the Mk2 you need a permanent battery feed to the "Memory" connection on the sled, and the ignition switched 12V goes to the "Ign Sense" line. This is the main power feed to the Mk 2: hence, I'm not too sure that the feed point you have chosen is really the best. It really needs to be a feed derived from the ignition switch.

If you have any Mini related intall issues, mail me through the board.