Once again, just writing the question down solved it. Since the setter is NEMO, there is an ambiguity as to whether NEMO or ME gets dropped...if one then not the other, so the M in the NW corner does double duty...
As ever thank you Meursault - for some reason I had ELO and instead of EDO amd I am not a prog rock fan! As the title suggests you get NEMO out of the phrase but leave ME in.
Just one more query (although it does not really matter because I do not submit entries) I seem to have a letter at 38 and another at penultimate cell of the middle row that appear to be superfluous (except as part of answers to conventional clues)?
I have a full grid, all the thematic entries, but I can't get clues 17 or 18 to convince me that I have the correct solution. Nevertheless, a stunning puzzle.
Thanks Demeter for that one. Really having a struggle with this Xword so many cold solves to deal with . No idea about the theme either . I'll keep plodding on for a while !