Round Robin XII

Eight answers, symmetrically placed in the grid, are to be thematically modified before entry.
The wordplay in all clues indicates the answer with an extra letter that is not entered in the grid. In clue order, the extra letters spell out two names and something which the unclued down entry (three words) eventually realised. Solvers must highlight a three-word message which was put on one side (14 cells). (Solvers are advised to search the theme using just the names.)
Numbers in brackets refer to lengths of grid entries.
The Chambers 13th edition is the primary reference.



Across
  1 Elderly bless bog near Argos, round back of store (5)
  6 Thrashed in retaliation Nicodemo, murdered in hiding (7)
12 Recurrent element in quirky B-movie conveying a warning (6))
13 Anxiety inside of head almost invisible on the outside (6)
14 Following a plan, Svengali's originally, old alchemist acquires occult knowledge (9)
15 Accolade for wine: dry with traces of tannin and elderflower (7)
17 Religious series backs even bits of Danny Dyer’s product (5)
18 Dressing expertise found in Dali’s homework (7)
19 Film producer casting actor when Spielberg movie's wrapped (6)
20 Before I hit on the head she had driven away (6)
21 Deeds Catholic bishop loses moving houses (7)
26 Perfect, is she?  You'll need to decide about that (6)
28 Handsome street endless nouveau riche ultimately ruined (6)
29 Plant's very big and ordinary for the most part (7)
31 Extravagant grand-niece neglecting heartless grand-uncle could be more sparing (5)
33 Butterfly DNA, on reflection, is what distinguishes an admiral for example (7)
34 Individuals choosing clubs to run into personalities (9)
35 Sporting a tie in a band (6)
36 Drive at Troon? Send it up middle of fairway, where leaves are often found (6)
37 Collapse of the core ingredient of Christmassy stollen (7)
38 Poet's despots lark about around the end of Comus, Scene 1 (5)
 Down
  1 Amphibian and wild cat run amok (7)
  2 Bad-smelling runny stool probed by American serving as disease expert (10)
  3 Referring to recurrent wind, I stand struggling with first signs of embarrassing
     emissions (7)
  4 Bits of African man discovered in Leinster, oddly (7)
  5 A drink mixed in decanter with no lid (6)
  7 Dance the French way (bustle raised!) (5)
  8 Mouthy suspect initially withholding name - that’s not right (7)
  9 Tipple served up during feast fit for flunkeys? (4)
10 E Izzard in marathon runs on, gone barmy, crackers? Not so! (6)
11 An accomplishment to get beginners to set clues, keenly knocked out (6)
16 Regular rover loses heart to beauty (10)
22 Plant genus of cabbage, eggplant and neeps initially caught up in endless Soviet revolution (7)
23 Look for dictator and prophet in a sort of tangle (7, 2 words)
24 Bennett girl acquires sign of sunbathing after beginning to try orange colorant (7)
25 Clued-up jockeys dismiss principally inactive ranch workers (7)
26 Palms of some meditative folk brushing off thanks (6)
27 Joyful expressions of fielders now and then after result (6)
28 It might pop new romance (6)
30 Key on instrument -- old man sustains it with heel on pedal (5)
32 Braided rope being brought aboard isn't abnormal (4)

Entries

To enter this competition, send your entry as an image or in list format giving the final entries to ccpuzzles@talktalk.net before 8th May 2020. The first correct entry drawn from the hat will receive a book from the Chambers range, which has been donated by Chambers.

Voting
You may vote for your favourite clue with your entry. You can just nominate one clue, which will be awarded 3 points, or your top three which will be awarded 3, 2 and 1 points. The person whose clue accumulates the most points will receive a prize, which also has been donated by the Crossword Centre.

Credits
Theme: Ian Simpson. Grid: John Nicholson
Clues: Steve Bartlett, Rod Beards, Rod Bell, Shirley Curran, Andrew Fisher, Richard Foden, Derek Harrison, Richard Heald, David Hennings, John Hood, Margaret Irvine, Nebuchadnezzar, John Nicholson, John Nolan, Mark Oshin, Bhalchandra Pasupathy, Antony Percy, Rob Pinnock, Phylax, Ed Powles, Sowmya Ramkumar, John Reardon, Rishikesh C G, Simon Shaw, Ian Simpson, Mona Sogal, Sprout, Hamish Symington, James Taverner, John Tozer, Vismut, Ajeesh V M, Luciano Ward, Nick Warne, Clive Weatherley and Keith Williams.