Thanks Sudokulover - I had pencilled that in ages ago but was too stupid to see the wordplay!
My 9 down is now obviously wrong!
I have never heard "IMPROV" as an abbreviation but I have just looked it up in Chambers and it is there, of course.
6 down now restored to my original answer and
EUREKA! All now slotted neatly into place.
This is the most difficult puzzle for ages in my opinion and without a little help I don't think I would ever have completed it.