Duncan, Unfortunately unlike the TR and Series one cars like you I have a cat on our Plus 4, when I get round to it I swap it for my cat link pipe which is a straight swap. I have had no problems with getting the car through the mot but there again its a 400 cell, if you have the software (like I do for my car) there is a setting to lean or richen the mixture which may be of use, but do store old factory default settings first!
