I found this which states the law. No looking good if you want it to pass its mot in a few years
Type Approval 94/20/EC - Mechanical Couplings
The introduction of a type approval procedure for mechanical couplings is intended to harmonise standards of their construction. It should also ensure that, in concert with other whole vehicle type approval provisions, the fitment of couplings complies with the recommendations of the vehicle manufacturer in terms of the most appropriate mounting points and methods of fitment to help avoid damage to the vehicle.
The measure will also contribute to the harmonisation of standards throughout the EC and assist in achieving a single market. UK and European manufacturers of such equipment will compete on level terms with regard to performance and construction requirements.
"Use of mechanical coupling devices
96B (1) This regulation applies to every light passenger vehicle first used on or after August 1st 1998 in respect of which an EC certificate of conformity has effect.
(2) No person shall use or cause or permit to be used on a road any vehicle to which this regulation applies unless any mechanical coupling device which is attached to it complies with the relevant technical and installation requirements of Annexes I, V, VI, VII of Community Directive 94/20(a) and is marked in accordance with Annex I to that Directive."
The Translation
Any light passenger vehicle registered in the UK on or after August 1st 1998 (S Registration) will require a type approved towbar and towball (when fitted).
Light Commercial Vehicles and Car Derived Vans are not covered by Type Approval. (LCVs continue to be exempt and are unlikely to be covered for the next 5 years or so).
This legislation will not be retrospective, therefore any vehicle registered before August 1st 1998 will never require a type approved towbar (e.g. R registered vehicles will never require type approved towbars).
Any vehicle requiring an approved towbar also requires an appropriately approved towball. Towball D and S values must match or exceed the towbar D and S values.
Type Approved towbars and towballs can be identified by the presence of a Type Approval label which must be visible even when the product is fitted to the vehicle. This label will have as a very minimum, the product code, a D value, an S value and a European approval number in the form e11 00-01234. These labels must not be removed or defaced as this will invalidate the towbar manufacturers warranty and would result in the towbar or towball not being identified as Type Approved.
[Edited by chrhodes on 28 June 2005 at 9:21am]