
Want to scare me straight off the bat? "Some of the across clues are "Letters Latent"". Crap. My least favorite form of clueing (I'd almsot prefer printers devilry). And only some of them, great.
My memory is a little flaky, but I think I tried one Lavatch puzzle before ("Fallout") and it was nearly an empty-gridder. What else do we have here...
Down clues are two clues side by side (and each has the same number of letters) with a separator word.
Well so much for my brilliant idea of working on the down and eaving etters atent unti ast.
Breathe easy, the first across clue isn't letters latent, it's A,B in G,BED. The second one looks like a definition of Noble and a Frenchman in P-E, Rene is too short, Alain? PALA(T)INE. I solved a letters latent! And I have a first row. 11 looks like it could be AIR(H)OLE and maybe these letters latents aren't going to be too bad. 12 is a straight out ODENSE. Thinking I'm going to get a lot more of these latent lettery ones, I attack the rest of the acrosses. I like 20 - SA(P)ONIFIES, 33 is a non-latent DAHLS, 36 is a nearly giveaway (H)EDGING, and 38 looks like some anagram of SSSNAT. I've got a decent part of the grid filled in!
Can we guess at some downs?
2,25 - I can see the first half is AIRMEN, which has to be 2. So NUCLEAR is the extra word
3,19 - I can see the second half is BIOLYSIS (Lavatch must be a scientist), which has to go in 19. CALLED is extra
4,24 - some atlassing tells me that PINEGA is a Russian river, and has to go in 24. IN extra
Having the first letter of most of these downs is helping.
5, 32: First half looks like GAES, the other half then has to be DE-- - DEAN?
6,30: POLIO is the rather uncomfortable second half (and fits the first two letters at 6).
7,28: Finally one where both halves appear - LE(H)AR and VIS,OR!
8,27: ANISE (fits 8), INFOLDS is extra.
9,26: WEALD is the second half (remember that one from Carte Blanche!), goes in 26.
10,13: TETRAHEDRA (more science!) is the second half, and goes in 13. A director starting with E and having 10 letters has to be EISENSTEIN, let's write him in at 10. OBVIOUSLY is probably the link word
14,21: ummmm, no clue
16,22: The second part is an anagram of SALE meaning a woman, ELSA or LESA, either would fit at 16. or 22...
This is looking more promising, back to the lers laen. 15 is HI--S which probably means HINGES without an E, and little cokehead Martina Hingis is our tennis player. And hello - this row reads TRIVIAL-HINGS. If 14d starts with a T we've found our phrase!
17 is (R)EASSU(R)E - E ASSUME minus the M.
Penny drop moment at this point... there's four whole words in those across clues, and so far it seems at least three of them don't have letters latent! THER-P-O--H- got to be THE something OF THE. RAPE? Google tells me that THE RAPE OF THE LOCK is a poem by Alexander Pope and it contains the words "What mighty contsts rise from trivial things"!!!! The first letters from the down clue words have to make an 11-letter word that comes before LOCK. I have NCIAMITO... COMBINATION!
Very excited by this find, I bashed out the downs... the B has to come from BRITAIN in 14,21. TENNO is the first part, and confirms my idea at 14d. Chambers Word Wizards suggests BRIAN ENO as a possibility for the letters in 3D (Hey, Brian, you're in Word Wizards!), wordplay words, BRAIN(move the A), E,NO. Knowing I need a latent A gives me TENAILLON (and suggests 4D wasn't BOVINE, but BOVRIL), and ANN(E)XATION for 23.
I have a completed grid!!!!! Now what... replace TRIVIAL THINGS with ALEXANDER POPE of course. That solves... nothing. Something's got to appear in another row.
Looking at the grid, head-smack moment... there's an X in the fourth column. It's a COMBINATION LOCK, I have to move the columns, and rather neatly, there's only one L in the second column, one E in the third and so on. Our three ambiguities are that there's three A's in the first column, two E's in the 8th and three E's in the last.
I printed off a new grid and wrote in the new columns and saw... nothing. Four rows looked like possibilities, so I wrote them out again, with all the possibilties for the ambiguous letters

Still nothing... back to the preamble... the theme's last line. Oh, what's the last line of the poem...
"And 'midst the stars inscribe Belinda's name". There's BELINDA, and if we choose our combination write, she's in GEMINI.
Lavatch - this was hard and fun. I spent lots of time on it, and where initially I was dreading it, the lttrs latnt didn't turn out to be so bad. This is one of the most involved Listeners that I've gotten to the bottom of, many many steps.
This is a long long report, but I really liked the puzzle. And I got it! Another point to George in the battle and a slight lead is restored.
Current tally: George 15, Listener 12. Current streak: George 2.