thanks fo those replies - I will certainly try turning on the heel instead.
I had been to a knee specialist before (several years ago), and they explained various things about my knee and what was happening, as well as strengthening excercises - however they couldn't provide advice on the best way to do aikido! However, this small bit of info. may stop deterioration whilst still allowing me to turn well! (I must admit, I used to do hundreds of tae-sebakis and irimi tenkans 'till I felt dizzy - now I just do a few good ones).