Many thanks to Geeker @44, Brendan @42,44 and 45 and Minno at 51
I've finished now. I'm not sure 24ac is a familiar name to me; I didn't know the abbreviation for "mark", though I had identified the answer as a possible. I was completely misdirected by 25ac, so thanks for the hints.
In fact, thank you to everyone for your input. My FOI was 4ac and my LOI was 19d. My CODs were 4ac, 6d and 16ac.
I haven't even had my glass of white wine yet.
I'll be back next week.