Just found this on the VOSA website
http://www.dft.gov.uk/vosa/repository/technical pen picture 4.pdf
Section 7.1 – Exhaust system
The major change in this section is that a catalytic converter missing becomes a reason for rejection. This is likely to be a major factor for some vehicles first used before August 1995 that only require a non-cat emissions test, but were originally fitted with a catalyst.
http://www.dft.gov.uk/vosa/repository/technical pen picture 4.pdf