Since February 2014, where a vehicle should have a diesel particulate filter (dpf), a visual check for the presence of the dpf has been part of the MOT.
Owners who are found to have removed the dpf, face fines of up to £1,000.
In May 2018 the MOT is being strengthened to determine whether emissions controls have been tampered with.
http://www.bbc.co.uk/news/uk-41761864