When I bought my K-Series 7 it was fitted with a non-cat exhaust so, to get it through an MOT I bought and fitted a 'proper' one.
Later on I took the old one to my friendly local welder who opened it up, removed most of the contents and welded it up again. When I got it home, fitted it and fired the engine up, I made young children cry and frightened several old ladies
You will see from the pictures that the cut section is on top of the can so, with a guard fitted it is invisible. Should you wish at any time (ie MOT) to take it off and replace it with a compliant one, it takes no more than 5 minutes: two bolts on the 'C Clip' and one on the side support bracket.
Now that I have sold my 7 I will not be wanting it anymore. Since a full K series exhaust is well over £200 and I paid £100 for the welding my asking a mere £50 is ridiculously cheap but you will have to collect it from Caterham. The town not the car.