Hi quocunque,
I came to this puzzle late today (NZ time), but 18d is first four letters = blemish then "about and 1" in reverse.
I thought 21 and 22 were really clever - seriously groaned when I got 22. I found it really hard to pick a COD because I liked so many of them, but I'll go for 12a and 24a with a highly commended for 26a.
Re 19d, I managed to guess the playwright from the crossers once I had the PDM on 24a - I too love the split clues.
I also had to research 23 across because I could work out the lake from the clue but had no idea there was a tribe named after it.
I also had never heard of the second def for 17a.
Toughest one for me was 1d because although I'd heard the abbreviation before, I'd never heard the clue reference to it before - and I'm ashamed to say I lived in London for 7 years so should have heard it along the way somewhere.