Thanks for the links. Sorry to keep bothering you but are you sure it's the same for the 124 as the 124G? (I have the latter). Here is a shot of the rear taking lens element on mine taken from the inside...
I don't know if you can tell from that picture but the space between the various parts is so small that I can't really get any tools in there (at least not without scratching up the whole inside of the camera), definitely not a spanner unless it's a tiny one. I could just about get some needle-nose pliers (wrapped in rubber) onto the outer barrel but trying to turn it had no effect. I can also get both thumbs inside the barrel but again it didn't feel like it was supposed to turn. The two very small slots you can see on either side of the inner ring are why I was asking whether a special tool was required.