Thanks Ginge.
Re 18, I'd split the clue 2 letters earlier, taking the last word of the clue as a 2 word definition. Doh!
Re 10 letter word, thanks for confirmation of final square. I'd actually seen that 5 letter prefix but couldn't see how to extend it to a 10 letter word so thought I must have gone wrong in the final construction. I can now see the answer, which I'd never heard of but, for some reason, never thought of looking up in Chambers. Another doh!
Thanks again. I can now get on with my life.