I didn't have a huge amount of difficulty filling the grid, but I'm currently facing several loose ends. I wasn't familiar with the thematic material but managed to figure it out, and I have the "set"—but for the life of me I can't seem to uncover the symmetrically-disposed pattern described in the preamble, in order to actually circle the relevant letters. I'm not sure what I'm missing here.
I have also failed to fully parse several bits of wordplay, more than usual. I am confident in the definitions. Nudges welcome:
15ac: I have most of the wordplay but I must be failing to correctly parse an indicator or something like that, I can't make it work tidily.
34ac: This feels like very straightforward wordplay except for a single letter for which I cannot account.
19dn, 27dn: At a loss here.