Right, Ive managed to sort it now, its not the most elegant solution but it worked.!
The problem was the grub screw has partailly become unscrewed from the hub and got wedged between the hub and the cassette locking ring. This meant that when the cassette turned the lock nut on the hub was tuyring as well, so when the wheel was put back on the bike and the srcews done to a sensible tightness the wheel wouldnt turn! (does that make sense? It does in my head) So the grub screw isnt accessable and cant be screwed in or out any more
Solution: get a hack saw and saw out a small portion of your cassetee removal tool thats big enough to fir around the grub screw (se. You can then remove the cassette locking ring and access the grub screw, either removing it completely or screwing back into place properly (with some locktite this time!). If you only take a small protion out of your CRT then it still works fine, and even if it now doesnt last as long its cheeer than buying a new hub or cassette!