Just found this on a forum, it is an extract from the highway code.
223: All vehicles MUST display parking lights when parked on a road or a lay-by on a road with a speed limit greater than 30 mph.
Law RVLR reg 24
224: Cars, goods vehicles not exceeding 1525kg unladen, invalid carriages and motorcycles may be parked without lights on a road (or lay-by) with a speed limit of 30 mph or less if they are
at least 10 metres (32 feet) away from any junction, close to the kerb and facing in the direction of the traffic flow
in a recognised parking place or lay-by. Other vehicles and trailers, and all vehicles with projecting loads, MUST NOT be left on a road at night without lights.
Law RVLR reg 24
strangely though sec 24 RVLR does not mention 30mph & it wold seem his trailer should not be left once detached