Engine Cooling System problems of the 2002 Mini Cooper S

I have a 2002 Mini Cooper S. The car?s engine cooling system has ongoing issues, particularly the coolant expansion tank. I have had to replace the coolant expansion tank three times within approximately 50,000 miles. This means my car is now on its fourth coolant expansion tank. Here is a short summary of the coolant expansion tanks that my car has had: date tank installed date tank failed/replaced car mileage when replaced when car was built June 19, 2004 14,130 June 19, 2004 October 23, 2004 17,528 October 23, 2004 October 24, 2007 50,143 October 24, 2007 not yet not yet the radiator also failed at 50,143 miles, which is an event I am also not pleased with. Additionally, it now appears--as of September 2008--that I may have another cooling system issue. I will be calling my mechanic soon to check into it. All of these failures of the coolant expansion tank took up substantial amounts of my time in taking my car to the repair shop and then going to retrieve it after the repair. And in one case left me stranded on the side of a very busy California freeway at rush hour. Additionally, I fear that the multiple failures of the expansion tank have stressed the other parts of the engine's cooling system (as evidenced by the radiator failure and the need to replace another coolant part). The local Mini cooper dealer replaced the first two expansion tanks under warranty and declined to replace the third expansion tank. After I complained--and after six months of follow-up calls and letters--they finally reimbursed me for the cost of the third expansion tank. In all instances, Mini declined to let me keep the old part--even though I asked each time.