DFT/VOSA only have 2012 testing guides available (PDF) on their gov.uk site.
As far as I can tell the amendments to do with exhausts were for public service, HG and agri vehicles. They can fail for having an incomplete exhaust, which was amended this year by providing a better description of what makes a complete exhaust (so to include cats, silencers, etc.).
As for passenger cars, as Pushki said there were changes. They are active from tomorrow (20/3/2013), although DFT/VOSA say this will be updated in the testing guides later this year.
Under 2013 special notices this is all that is said about exhausts:
Method of Inspection amended for purpose of clarity, to:
On petrol engine vehicles that qualify for a full catalyst emissions test, check
the presence of the catalytic converter.
Note: To ascertain whether a vehicle qualifies for a full catalyst test, use the
flow charts in Section 7.3.C disregarding the result of the Basic Emissions Test.
So if it's meant to have a cat and it doesn't, it fails. Pretty sure that was how it was anyway? The chart in section 7.3.C is of course the Jan 2012 one that newkid posted.
http://www.dft.gov.uk/vosa/repository/S ... 20V1.0.pdf