I have a 2003 BMW Mini with 40,000 miles on the clock. Towards the back end of last year, it developed a rattle which was subsequently identified as the catalytic convertor substrate failing. BMW quoted £1100 to replace it, which I thought was very expensive, particularly as the cat' has only lasted for about half of its typical usable life. So I raised it with BMW UK, unsuccessfully. My query is this - is there a case to answer, or do I take the hit and shop around for an aftermarket catalytic convertor which would be a lot cheaper?