This is the C&U regs:
1. Number (on each side of a vehicle)–
(a) A motor vehicle with three or more wheels, not being a motor bicycle combination, first used on or after 1st April 1986: One front indicator (Category 1, 1a or 1b), one rear indicator (Category 2, 2a or 2b) and one side repeater indicator (Category 5) or, in the case of a motor vehicle having a maximum speed not exceeding 25 mph, one front indicator (Category 1, 1a or 1b) and one rear indicator (Category 2, 2a or 2b).
(b) A trailer manufactured on or after 1st October 1985 drawn by a motor vehicle: One rear indicator (Category 2, 2a or 2b) or, in the case of a trailer towed by a solo motor bicycle or a motor bicycle combination, one rear indicator (Category 12).
(c) A solo motor bicycle and a motor bicycle combination, in each case first used on or after 1st April 1986: One front indicator (Category 1, 1a, 1b or 11) and one rear indicator (Category 2, 2a, 2b or 12).
(d) A motor vehicle first used on or after 1st January 1936 and before 1st April 1986, a trailer manufactured on or after 1st January 1936 and before 1st October 1985, a pedal cycle with or without a sidecar or a trailer, a horse-drawn vehicle and a vehicle drawn or propelled by hand: Any arrangement of indicators so as to satisfy the requirements for angles of visibility in paragraph 3.
(e) A motor vehicle first used before 1st April 1936 and any trailer manufactured before that date: Any arrangement of indicators so as to make the intention of the driver clear to other road users.
And this is the MOT requirement for post 86 only:
Vehicles first used on or after 1 April 1986 must be fitted with one side repeater indicator on each side. Instead of a separate lamp, the side repeater might be part of the front direction indicator if it includes a wraparound lens. Lamps incorporating a side repeater are marked either with an ‘E’ mark in a circle or an ‘e’ mark in a rectangle above which is a number 5. However, some vehicles are fitted with a wraparound lens with no European approval markings. These can be tested by standing approximately 1000mm to the side of the vehicle’s rear bumper with the indicator on. If amber light can be seen coming through the front lens (not a reflection) this is acceptable.
Luckily, I have a sensible MOT inspector
