41 to 5 by Schadenfreude

This was an amusing and eminently solvable puzzle from Schadenfreude, a reliable old friend. The "gimmick" of placing numerical digits (and a decimal point) in some cells was here put to good use in service of the theme, helping to spell out the conversion formula from degrees Fahrenheit to Centigrade. Following this hint, the solver found the word "Fahrenheit" centrally placed in the grid, and replaced it with "Centigrade" (which happily has the same number of letters), forming an appropriate letter "C". To Gregson's shame, even when the formula was revealed in full it took him a few seconds to cotton to its significance. Most solvers probably saw the theme a mile off. But just solving the clues provided pleasure enough, with a neat penny-dropping moment when all was revealed.

Across
1 Transport retired couple to annex (7)
SUBJOIN (BUS rev. + JOIN)
6 Certified idiot died following operation (7)
ASSURED (ASS + URE + D(ied))
12 Remove the fastening from one leg (5)
UNPIN (UN + PIN)
13 Venture capitalist's first sci-fi film about the universe (6)
CALLET (ALL inside C (first letter of CAPITALIST) + ET (film title))
ET is a sci-fi film?
15 Tidy pile up (3)
BIG (Two meanings)
16 With these dry fruit portions she can make exotic choc-ices (5)
COCCI (Jumble CHOC-ICES minus SHE)
17 Very soft unionist flanked by first class support (5)
APPUI (PP (pianissimo) + U(nionist) inside AI)
18 Turn out without old politician (4)
TORY (O(ld) inside TRY)
20 Victor chasing commander to fir with numerous warheads (4)
MIRV (MIR (commander) + V(ictor))
21 Injured international dropped by United (6)\
MARRED (MARRIED minus I(nternational))
22 Young men extremely hostile pursuing old record by AC/DC (6)
EPHEBI (EP (old record) + HE (first and last letters of HOSTILE) + BI (bisexual))
23 Theatre finally passes with revised germ-free conditions (7)
ASEPSES (Jumble PASSES + E (last letter of THEATRE))
25 A girl on good terms with Queen's third son (4)
INES (IN + E (third letter of QUEEN) + S(on))
28 Water-buffalo's love for a river (4)
ARNO (ARN + O (love))
30 Satellite echo interval (7)
TRITONE (TRITON (moon of Jupiter) + E(cho))
33 One is getting into the fancy dress (6)
TUSSEH (US (one) + S (is) inside jumble THE)
35 Servant's pocketing copper coin (6)
MANCUS (CU (copper) inside MAN'S)
37 Disputed tie up with Perth (4)
THREEPIT (Anag. TIE PERTH)
39 Not a rule in northern yard (4)
NARY (A + R(ule) inside N(orthern) + Y(ard))
40 Look after wife in best upstairs room (5)
TWO-PAIR (W(ife) inside TOP + AIR)
41 Coloured edge on special curtain material, perhaps (5)
SCRIM (S(pecial) + C(oloured) + RIM)
42 William's eager sermon (3)
PRONE (Two meanings)
43 Your book's first yarn (6)
ZEPHYR (ZEPH (Book of Zephaniah) + YR (your))
44 Drama unfolding in top motor sport (5)
AUTOPOINT (AUTO (drama) + jumble IN TOP)
45 Orkney Islands can produce ladies with this language (7)
NYNORSK (Jumble ORKNEY ISLANDS minus LADIES)
46 Something light from page nine - why get excited? (7)
PENNYWEIGHT (Jumble P(age) + NINE WHY GET)

Down
2 University is not checking on clubs for students (6)
UNIONS (ON inside U(niversity) + NIS (is not))
3 Page in journal reflected long-winded talk about source of fibre (7)
JIPYAPA (P(age) + I(n) + J(ournal) rev. + YAP + A(bout))
4 Iron men ending in confusion score badly (6)
ONCERS (Jumble SCORE + N (last letter of CONFUSION))
5 One to show a picture (4)
ICON (I + CON)
7 An imprecation has sparkle (5)
'SLIFE (S (has) + LIFE)
8 She apparently goes in to apprehend a big fish (6, two words)
SEA APE (A (she) + AP(parently) inside SEE)
9 College abandons development of precision instruments (8)
RIPIENOS (Jumble PRECISION minus C(ollege))
10 Excursion to ancient city's crossing black residential areas (6)
EXURBS (EX(cursion) + B(lack) inside UR'S)
14 Long letter from Shakespeare (4)
ACHE (Two meanings)
19 A blocked passage (4)
ADIT (A + DIT)
21 Jesus has space for Henry Olivier, the organist and composer (8)
MESSIAEN (MESSIAH with EN replacing H(enry))
24 Outstanding quantity of money for the auditor (4)
SOME (Homophone SUM)
26 A weapon wounded him (7, two words)
STUN GUN (STUNG + UN)
27 Source of water for example above bend (6)
SUPPLY (SUP (above) + PLY)
29 Badly behaved horse placed behind Queen (6)
REARER (REAR + ER)
31 Raffles perhaps in charge with rising authority (6)
ICEMAN (IC (in charge) + NAME rev.)
32 There's little new with any old contract (6)
NARROW (N(ew) + ARROW (any))
34 Hinged plates wrap round square section (5)
HASPS (S(quare) inside HAP + S(ection))
36 Jaunty, flashily dressed, heading off (4)
AIRY (LAIRY minus first letter)
38 Apparel to wear out (4)
TIRE (Two meanings)