I found a site that gave allowable overall lengths for 'road trains' with figures for multiple trailers - but I cannot resolve who is allowed to use such combinations.
Quote:
Regulation 7 Road Vehicles (Construction and Use) Regulations 1986 states that the overall length of a vehicle or combination of vehicles shall not exceed the maximum length specified below :-
Combinations:-
* motor vehicle + 1 drawbar trailer 2 - 18.75 metres (see Note 3 below)
* showman's motor vehicle + 1 drawbar trailer where the trailer is living accommodation - 22 metres
* motor vehicle + 2 trailers - 25.9 metres (motor vehicle 9.2 metres max, only one trailer may exceed 7 metres),
* motor vehicle + 3 trailers - 25.9 metres (motor vehicle 9.2 metres max, no trailer may exceed 7 metres),
* articulated bus and a bus drawing a trailer - 18.75 metres (from 1/4/2003)
* articulated vehicle + semi-trailer (see Note 2 below) - 15.5 metres, (see Note 4 below)
* articulated vehicle + semi-trailer which is a low-loader manufactured on or after 1/4/91 - 18 metres. (see Note 1 below)
* Note 1 - Except trailers and combinations including trailers constructed and normally used for abnormal length loads or plant trailers for road construction.
* Note 2 - Except combinations involving towing broken down vehicles or trailers constructed and normally used for abnormal length loads.
* Note 3 - If motor vehicle manufactured before 1st June 1998 - 18 metres unless certain conditions apply - until 31/12/2006.
* Note 4 - Up to 14.04 metres trailer or 16.5 metres combination (16.69 metres for car-transporters) if semi-trailer meets turning circle requirements (Regulations 13A or 13B).
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http://www.transportsfriend.org/road/dims.html