Wow! What an absolute beast of a puzzle by Paul.
In addition to there being a theme, there is, as Geeker says, a large amount of of GK required or, in my case, several trips to Google!
If you have trouble honing in on the theme, I recommend 12a as a possible way in and that should also help you get 26a.
FOI was 8a but it was at least 15 mins before I got another one.
If you do get stuck, there is a long anagram in the downs but it's not in Chambers or Collins.
I still haven't parsed 1d and am not at all sure I have 5d correct but, like Geeker, my head hurts so I'll revisit them tomorrow with fresh eyes.
With the possible exception of 25a I didn't notice any Pauline clues but, in truth, I wasn't really looking.
COD - A possible side effect of themed puzzles is that answers sometimes have to be crowbarred in and that can adversely affect the wordplay which, I think, is the case here but, that said, I'll go with 23a.
Thanks to Paul for a very challenging puzzle.
Please feel free to leave a comment and, as always, if you get stuck - don't hesitate to ask for a hint.
Stay safe:-)