Up Date,

I have one set at home and I've experimented with various method of softening or dissolving whatever was causing the friction, as suggested by various websites:

VInegar, no effect
Citric acid, no effect
Caustic Soda, don't try this, everything just fizzed, and it started to dissolve the aluminium.
Heat on it's own, no visible effect.
Heat followed by tapping the sheaves with a soft rubber mallet to move the sheave axially along the shaft, ie not trying to rotate the sheave, but trying to get some end float. This worked almost immediately, as soon as the sheave moved axially, and there's only a couple of mm of end float, the sheave freed up, and after cooling, the sheave was free to rotate. I used a hot air paint stripping gun, and I guess I heated the sheave to just above 100 deg C. Initially the water boiled on the surface of the sheave when I put it under the cold water tap to cool it.

I quickly freed up the two sheaves on the organiser I brought home, however, there are many more to do when I get back to the boat in about 10 days time (the boat is 140 miles away).

Thanks for all the input, and I hope others can find this post if they have the same problem.