Thanks,
I left the tape on all of the MF rolls, and some of the Magic tape I used for the bulkloaded Efke may have been left on.
The goo was dark (as dark as the emulsion in the developed frames) and it stuck to the clothespins, leaving marks and a clear spot of film base. Slightly slimy feel to my fingers when I touched it.
The frames in themselves look okay, apart from that they are taken in very different light levels and some are underexposed, etc.
All of the MF rolls has the residue. The goo is most pronounced on the roll sitting at the bottom of the tank, while the topmost is almost clean. Incidentally the topmost is a two year old roll, while the bottom one is at least seven years. Wish I had a scanner you could show it - a picture goes a long way of explaining.
It's no big deal - if I don't find any solution, I'll just move on and do some more, giving an even more thorough rinse, fix and wash.