Regulations 21 and 23
SCHEDULE 8Requirements relating to obligatory and optional hazard warning signal devices
Every hazard warning signal device shall—
(a)be operated by one switch;
(b)cause all the direction indicators with which a vehicle or a combination of vehicles is equipped to flash in phase;
(c)be provided with a circuit-closed tell-tale in the form of a flashing light which may operate in conjunction with any direction indicator tell-tale; and
(d)be able to function even if the device which controls the starting and stopping of the engine is in a position which makes it impossible to start the engine.