The new rules come into force this month but from what I can establish they only related to DPF filters, not CAT's. Also, it only applies if the tester can establish that the vehicle being tested was fitted with a DPF filter as standard, although it will probably be pretty obvious to him that the DPF has been removed unless more exhaust sections have been changed. Someone mentioned CAT on a 300 TDi Disco - not all 300 TDi Disco's had them, so the tester would have no way of knowing even if the legislation applied to CATs.
https://www.gov.uk/government/news/new-rules-for-mot-to-test-for-diesel-particulate-filter
I'm not sure what the experiences are with other people on the forum but the MOT test has got so extensive now, every car I have had tested in the past few years they have overlooked many items on the test - for example they are supposed to check the operation of the tow bar electrics socket if fitted but I have never had one tested... The only time I have ever heard of one being tested was my mate who took his Series III ex MOD FFR (Fitted For Radio) to Kwik Fit and they were using a trailer lighting board to do the test - his 24 volt electrics finished it off.