Thanks for your info Adriano. Sounds like it's a job for the experts, I reckon if I started to disassemble it I may bite off more than I can chew. It doesn't look like fungus to me, I've seen plenty of that in some old lenses I own. This looks like water dropplet stains. It's hard to establish which surface has been affected. I'm reluctant to clean the surface of the rear lens as I may make it worse. I'll post a few photos I took of it and see if anyone agrees with my assumption that it's caused by condensation water droplets.
