Me, I knew.On reading the IVA book of words, apparently an E3 marked light, with triangular reflector, is an automatic fail as triangular reflectors can only be used on trailers... if you see a triangle in the distance, you can be safe in the knowledge it's a trailer! Who knew...
Also having the triangle reflectors the wrong way up is also b******g from a Traffic Cop.
( Guess how I learnt... )