Thanks guys.
Duncan's point (via the helpful Police Officer) regarding positioning the lightboard in such a way as to aid following traffic in judging the distance to the end of the vehicle (big lump of metal sticking out at around windscreen height) makes a lot of sense.
I think I'll chop the arms so that the lightboard sticks out as far as the end of the skeg.
Due to the ground clearance of the boat/trailer/engine, I tow with the engine fully 'up' with the towing support bracket in place and a prop bag on the prop/skeg.
In addition the weight of the engine is distributed across the transom rather than hanging off it.
The new 'correct width' lightboard has the correct lights/reflectors on it.
I have got some self adhesive orange reflectors to attach to the side of the trailer.
Boat trailers are exempt the part about having white/clear reflectors at the front of the trailer (because they narrow to a point, so don't really have a 'front') however, I have a clear reflector to attach to the front of each wheel arch, more to appease any over-zealous Traffic Officer than anything else!
Thanks again guys and I hope this is useful to others!
C.