Found it. Makes interesting reading looks like we are all find to do what we want. To land rovers.
Towbar EC Type Approval
EC 94/20 type approval regulations were introduced in the United Kingdom on 1st August 1998 as a part of the wider European Whole Vehicle Type Approval.
The regulations mean that for passenger cars registered on or after this date, if fitted with a towbar, the towbar must be designed and manufactured to EC94/20 standards.
An EC Approved towbar will carry a plate or other durable label bearing the EC approval number plus other information including the S-Value (maximum tested noseweight) and the D-Value (the test load as calculated by EC94/20 requirements - note the D-Value is not the same as the towing capacity).
The main requirements of an EC Approved towing bracket are that it is fitted to all of the towbar mounting points specified by the vehicle manufacturer, that it has undergone a rigorous testing procedure overseen by the Vehicle Certification Agency (VCA), and that production towbars are subject to Conformity of Production (CoP) to ensure that production towbars are manufactured to the same standards as required by the initial test towbar.
If the car* is registered on or after 1st August 1998 and you fit a towbar, it MUST by law be approved to EC94/20 and installed correctly according to the manufacturers' instructions. It is an offence under the vehicle Construction & Use Regulations to fit, use or permit to be used, a non-approved towbar on the vehicle
* The regulations apply to Passenger carrying vehicles up to 3500kg Gross Vehicle Weight. Light commercial vehicles are not covered by this requirement, so for instance a Ford Transit van is not required to have a Type Approved towbar even if registered after August 1998.
Does the towbar have to be professionally fitted?
The towbar must be installed correctly under the EC94/20 regulations. However there is no requirement for the towbar to be installed professionally, there is no qualification required under the regulations.
Provided the towbar is installed in accordance with the manufacturer's instructions, it is perfectly permissible for the owner or anyone else for that matter to fit their own towbar to their car.
By The Way: HMRC confirm that even a Landrover Defender 110 Station Wagon with either 5 seats of 7 seats is indeed classified as a commercial vehicle.