Thank you very much for your quick response and my belated reply but since contacting you I have not been able to re-create the problem in spite of following all my usual procedures!
I'm hoping that the problem has gone away but just to reiterate, when selecting Crop in either RAW or Developed image the appropriate boxes all appear ie. Overlay, crop etc. but the selected grid does not appear and if you try to move the handles at the sides or top the whole image shifts and the cursor still shows as a sort of selection icon not the normal four way arrow that would normally appear.
I'll keep checking.
Many thanks,
Roger