Link to Direct Gov.co.uk info .
https://www.gov.uk/towing-with-car/towing-equipment
http://webarchive.nationalarchives.gov.uk/20121015000000/http://www.direct.gov.uk/prod_consum_dg/groups/dg_digitalassets/@dg/@en/@motor/documents/digitalasset/dg_200824.pdf
.