The below is from the gov.uk website:
'From February 2014 the inspection of the exhaust system carried out during the MoT test will include a check for the presence of a DPF. A missing DPF, where one was fitted when the vehicle was built, will result in an MoT failure. A vehicle might still pass the MoT visible smoke emissions test, which is primarily intended to identify vehicles that are in a very poor state of repair, whilst emitting illegal and harmful levels of fine exhaust particulate. It is an offence under the Road vehicles (Construction and Use) Regulations
1 to use a vehicle which has been modified in such a way that it no longer complies with the air pollutant emissions standards it was designed to meet. Removal of a DPF will almost invariably contravene these requirements, making the vehicle illegal for road use. The potential penalties for failing to comply with Regulation 61a are fines of up to £1,000 for a car or £2,500 for a light goods vehicle.'
To me it's still a bit ambiguous as it only mentions DPFs - not CATs specifically.
Also, I'm not sure about this bit, 'to use a vehicle which has been modified is such a way that it no longer complies with the air pollutant emissions standards it was designed to meet'.
Does removing a cat on L-series cars mean they no longer comply to the emissions standards they were designed with originally??
Of course there may be another bit of paper that talks about diesel cats that I haven't found yet.