"Diesels don't need a cat to pass the mot,stated by VOSA themselves, so technically de catting a diesel is legal!"
As a former VOSA chief inspector I can tell you it is ILLEGAL
to remove a catalytic converter from a car that is fitted as per factory and manufacturer specifications and drive on UK roads.
There is a common misconception surrounding De-cats... A lot of people get both regulations confused and assume that they cross over
As per current emissions testing procedures, a diesel is only subject to what is commonly called a "smoke" or clarity test when presented for MOT inspection... However....
If a vehicle has been "De-catted" and driving on UK roads it is subject to EU/UK emissions equipment fitment regulations and MUST be present and working....
Just because an MOT test does not check emissions currently on diesels like a petrol vehicle it does not mean that EU member countries don't... UK regulations have to adhere to the EU regulations and come in line with them, thus that is exactly why a UK registered diesel vehicle has a catalytic converter fitted as per EU specifications...
Given the amount of traffic that crosses over from the UK to European road networks, it is no good being apparently legal in one country and not in another!
That is exactly the reason why this equipment is fitted as standard and that we have to enforce it.
The MOT test however, is a completely UK standard of examination thus the difference in level of inspection on the day of the test only and since the car is presented for test at a UK testing station for roadworthy standards are met, it does not actually mean that the car is legal to be driven, it has just met the UK standards at that exact time. The examiner can therefore not apply the EU required examination procedure, they don't know where you will be driving next.
All I can tell you is...
If you drive on UK roads and get pulled over for a VOSA roadside safety inspection, the examiner will check your exhaust visually during the examination then if in doubt, simply insert a bore-scope remote camera that all VOSA vehicles now carry and this takes a matter of 1 minute to check inside your exhaust system. We simply press the feed button and the camera system run along the length until it reaches an impasse at the catalytic converter ... If no converter filtration material is present, the owner/ driver will then be issued with a fine via a court appearance and the car could be impounded as per "presented for examination in an un-roadworthy condition" regulations and could be removed, impounded and destroyed.
Ignorance of the fact that a car has been de-catted by a previous owner is NO defense or excuse.
Anyone considering De-cats should look at other legal options.
.. Like a sport cat as an example which is legal in all conditions for a diesel.
Please, consider this before any "advice" regarding this matter is given... The law is often complicated and confusing... We don't make these laws, we just have to enforce them.