Well, that was a marathon for me! I finally finished last night, but I had the much-posted problems with 5ac, 5dn, 10ac and the NW quadrant in general, so I didn't look at it for a few days. Many thanks to all who posted replies to queries here, some of which got me on the right track.
Thanks to Banjaluka for a cleverly-constructed puzzle with an unambiguous, well-shaped area to highlight -- much better than the one I suggested the compiler of Listener 4681 had in mind to compensate for the missing words at the start of the thematic quotation!
I did, however take issue with the second sentence in the preamble: "Ten clues to non-clashing answers each contain an extra word, hinting at THE thematic material (47 cells…) that ALONG WITH two figures…". To me, this indicates that the two figures do NOT form part of the thematic material. I can see the reason for making the two figures separate as they are not covered by the two hints to the thematic source derived from the extra letter removed from 26 clues. The two figures, however, ARE required by the final instruction given in order to complete the puzzle.
In the second sentence in the preamble, removing THE from "...the thematic material…" and amending "...along with two figures…" to "... along with A FURTHER two THEMATIC figures…" would have removed this problem".
Dismounting from my high horse, I really liked the clue at 31ac. Having reviewed the grid again just now, I thought yet again that the answer must have been put there to aid the inclusion of thematic material. Looking into this notion, I discovered a dogleg Easter egg -- doglegg? -- beginning at the first cell of 24 ac/dn. Neat!
Finally, I've come unstuck with two puzzles during 2021 by not seeking help with parsing clues, so help with the following would be much appreciated:
10ac This must be a 'normal' clue. I can see the answer and I'm taking 'increasingly' as an indicator of a comparative, but what is 'Sturgeon's' role in the wordplay? Seems redundant to me.
30ac Am I right in thinking the third word contains an extra letter and the answer is an anagram of O(ld) + SOD?
4dn I'm taking this to be an '&lit' clue with the answer being a Shakespearean character, but surely another character better fits the Bill… (Geddit?)