Well, well, well........
I finally found and somewhat destructively opened a Z8 headlight unit today. The inside of the Polycarbonate is actually coated with some sort of solution. If I clean it with inert alcohol it becomes very sticky and crystalises. This coating is what is going foggy, not the lens and it certainly doesn't look like a result of condensation build up. Never seen condensation that became sticky!!
I feel sure BMW know all about this problem.....maybe an email to the manufacturer Valeo, will shed some light. The degree of fogging is being caused by the thickness of the coating hence the reason also it looks patchy when seen close up
Regards
David