Well the clutch is finally apart without any damage to the plates. As John.K said bearing races were pitted, but the steel cage was undamaged. As an experiment ( and I'm a tight bug**r), I've made a single piece split bearing out of Teflon that fits inside the cage. I lightly ground the races smooth but left the pitting and scraped the Teflon untill the spocket spun freely. It may or may not work in practice, but I've burnt no bridges and it's easy to return to original. One question though, I've always understood that cork clutches need oil to provide cooling but this clutch is in the air, so how does that work?