Thanks to a little help from @darla and @chrise, I've now finished. Phew.
LOI was 18d - after changing my solution to 16a (Crossword solver could produce nothing plausible for L*R*, and I hadn't been able to parse Cuckold anyway, so I reconsidered it!) I searched through the entire forum for mentions of 18, and eventually got the answer after finding the hint that the first 4 letters were thematic.
The sequence, after reading those helpful hints, was 3,1,4,7,9,2,12,16,17, 16 revised, 18!
FOI 13A
LOI 18D
COD ... perhaps 21, ingenious anagram nicely surfaced
And then it was time for the next one - though I don't look at it until daytime. Thanks, all, for help with this very tricky (to me) puzzle. (But it's been a busy week - village panto, WI Christmas lunch, thwarted trip to visit an exhibition without checking opening hours carefully enough, that sort of thing)