When my '03 Cooper hit 120K miles I decided to replace the original spark plugs. Having used Bosch plugs for years with excellent results I bought a set of Bosch Platinum 2+ plugs and installed them. My mechanic told me that Bosch plugs are not a good idea in this particular engine. There have apparently been cases in which Bosch plugs have worked loose and blown out of the cylinder heads while the engine is running, causing damage under the hood. I bought a set of NGK Platinum plugs and replaced them. Just wish I had known this before I bought the first set of plugs...

 

DIY guide for replacing your spark plugs and wires: Pelican Technical Article: MINI Cooper - Spark Plug and Cable Replacement

We also carry the NGK spark plugs for your MINI: Project 7: Spark Plugs and Cable Replacement - Page 1