Hi Stuart,
I've been busy since my first post...
First of all, I tried from the back. I undid the tiny grub-screw on the milled bit. That milled bit then unscrews. Under it are three more screws, one grub and two slightly larger. I undid them all, but couldn't get any further. I reassembled that bit, and turned my attention to the front. Although I don't have circlip pliers, I was able to shift the front collar using two jeweller's screwdrivers in tandem. Underneath are three more grubscrews set equidistant around the nose. If you uncrew these, the whole nose drops out easily into your hand.
I carefully cleaned the glass at the back of the nose with a microfibre cloth, then used a can of compressed air to give the rest of the assembly a good blasting. While there, admire the quality of the internal construction; you can see that the different focal length views are achieved by two L-shaped pieces of metal that move in unison when you rotate the selector ring.
I put it all back together, and voila! The original bit of cack has gone...
...to be replaced by a tiny shred of fluff...
C'est la vie. When I have time I'll "field strip" the bugger again and get that out too! At least now I know how.
Regards,
Bill