Having now tried a whole mass of bulbs - I am convinced that it's the lens itself that is the issue. It is the antithesis of directional.
I have an idea or two - BUT would need things I simply do not (currently) have access. HIGH RES 3D scanner - and rapid prototyping - plus a CAD designer. A spare rear cluster and maybe even a rear guard.
Basically I think I needs a new backplate making up (maybe in aluminium - heat sink) and the use of more directional LED units. I would then opt for a commercial grade and approved LED cluster - a standardised size one (i.e. Land Rover Defender). Though a non ECE approved one could be easily designed ONCE the basic plate assembly is defined.
I will not be adapting my rear unit. I have additional tail lights in the top case and additional flashing tail lights in my indicators. I was going to add lights to the side cases - but can see no way to run the wires neatly without drilling - and the cases are watertight - so that won't happen.