From another website ( North Limcolnshire Council ) :-
These regulations vary slightly depending on the age of the trailer and the width of the towing vehicle, but this is what is normally required. The trailer board needs; two red rear lamps, two stop lamps, number plate and light, two red reflectors, two amber indicators, one rear red fog lamp, two white forward facing reflectors. In addition, if the trailers are more than 2.1 meters wide, they also need white forward facing lamps on the trailer board or mudguards. The lighting board must be wide enough so the outer edge of the rear lights is within 150mm of the outer edge of the boat. This lighting board should be positioned within one metre of the end of the total overhang (which is often the mast), or otherwise the end of the overhang would have to be lit with an addition reflector and red lamp. The lighting board should not be more than 1.5 metres off the ground. The trailer also requires at least two amber reflectors down each side.