Just read an article in Motor Mechanics, seems the most popular car for needing aftermarket replacement cats is the 107/C1/Aygo. The cause is due to the engine temperature sensor going out of range meaning an over rich mixture with consequential soot and petrol vapour destroying the cat. Hydocarbon emissions then exceed 200ppm meaning an MOT failure. A normal reading would be around 50ppm. Replacement of the temperature sensor at every coolant change is reccomended.