I think i've finished the puzzle after a long struggle. I enjoy reading this group's thread when I think I'm done, to see what everyone says.
It's either my treat to myself for having finished, or my solace when I've completely given up.
I'm not quite sure about one of my answers. It doesn't involve an extra letter:
in the LH grid, 23d, I can't see the parsing. Maybe I've picked the wrong word as the definition. There are a very limited number of words which fit in here. 4 letters, A-L- .
Also, in the RH grid, 2d, I'm sure my answer is correct since it's one of those kinds constructed out of a specific set of instructions. And I do know what it means. But I can't figure out exactly what part of the clue really means it. A couple of things sort of mean it, but not precisely.
Unless I'm way off-base. Again, no extra letter. 6, 4. -E-R-E / -I-I (second word is Latin).
rabbitlover's description of the thread as Mornington Crescent is so accurate and so funny that I laughed out loud when I saw it.
And to that I say--Cockfosters!!! (sorry--not very mature, I know...) and Happy New Year.