hi im an authorised examiner and mot tester and as of the 1st of january 2012 a car that was originally fitted with a cat at maufacture wether petrol or diesl which vosa do not specify is presented for an mot with the cat missing is a fail so before removing the cat i would ring your local vosa office just in case the other way would be is to remove cat knockout all internals and replace then it loock like cat still there but with no restrictions