Ok, The solution, whilst not ideal does work.
Basically, you have to edit the library that consumes the Content Type. This works, but I'm not sure what effect this will have if you later update the content type parent again and push out the changes.
Ok, I've just tested this, and surprise surprise, as soon as you update the content type, the column gets returned to it's previous setting.. so not ideal, but it will get you out of a jam!
www.myfatblog.co.uktwitter.com/@cimares