http://www.police-law.co.uk/law/policelaw.nsf/0/12c8b041478125b880256baa005ad911?OpenDocumentRegulation : 86
Act : Road Vehicles (Construction And Use) Regulations 1986
Subject : Motor Vehicle Drawing A Trailer By Rope Or Chain
The distance between the vehicle being towed and the vehicle towing must not exceed 4.5m. If it exceeds 1.5m the rope or chain must be clearly visible.