Puzzles based on Sherlock Holmes adventures seem to be a fairly regular occurrence, but this was the first Gregson could recall that used the Musgrave Ritual as its theme. In retrospect the route to the treasure described in that story would seem to be ripe for crossword treatment.
The extra letters (and number of letters in extra words) in the clues spelled out "Sun over oak shadow over elm, N 10 and by 10, E 5 and by 5, S 2 and 2, W 1 and 1", which was almost word for word the route given in the ritual. However the solver's first problem in following the route was to determine the exact starting point, which could arguably have been the O or N of DAMSON. Next the solver had to establish the meaning of "10 and by 10". In the Conan Doyle story this was rather briskly and unsatisfactorily interpreted as ten steps with each foot, but that concept had no obvious parallel here. But the X in MAGNOX provided the key - we were to go "by" X = ten. Similarly the V (= versus) of AVER gave the cell to be skipped in the easterly moves. The eight-step path thus spelled out TGNDAIWF. Matching this string to SHERLOCK gave eight cypher pairs so that e.g. S and T coded to each other. It was then fairly easy to discern the shape of a crown spelling out LEWONES WDICEIK NEHALER (the mention of ten lines showing the outline of the treasure strongly suggesting a crown). This message decoded spelled out "Ancient crown of England".
Across
1 Unclued (8)
SHERLOCK
6 Throw sticks at as[s] in pit endlessly (6)
SQUAIL (QUA (as) in SILO minus last letter)
12 Exercises in d[u]ress more than a handful for Scots (6)
GOWPEN (P.E. inside GOWN)
14 Type of nuclear reactor, ki[n]d with polluting gas (6)
MAGNOX (MAG (= tease = kid) + NOX (nitrogen oxide)
15 Sto[o]p by eliminating a judge (5)
ADEEM (A + DEEM)
16 Pro[v]e awkward holding hard magistrate (5)
EPHOR (Jumble PROE + H(ard))
17 Necklace her[e], one providing a link to Shakespeare (7)
TORCHER (TORC + HER)
19 [R]eyes - European in Arsenal's heart (4)
EINE (E(uropean) + IN + E (middle letter of ARSENAL)
20 Bird soaring over the bri[o]ny in view a gale's ruffled (8, two
words)
SEA EAGLE (Jumble A GALE inside SEE)
21 Partners with quantity of land make g[a]in (5)
SNARE (S + N (Bridge partners) + ARE)
23 As[k]s prince before close of show wearing suit (7)
HALF-WIT (HAL + W (last letter of SHOW) inside FIT)
26 [S]nipper see finally slice tissue (6)
DIPLOE (DIP (Pickpocket) + LO! + E (last letter of SLICE))
28 Biological material involved in whiplas[h] mainly (5)
PLASM (Hidden in whiPLAS Mainly)
31 Sort of silk [a]t banks (5)
TASAR (T + ASAR)
32 Film star ga[d]s around El Salvador (6)
NEESON (ES (El Salvador) inside NEON)
33 Criticising money acquired by tar[o]t (7)
SLATING (TIN inside SLAG)
35 Fairy accessories being new [w]in lots of money (5)
WANDS (N(ew) inside WADS)
38 Develop covering for o[u]r grove dreadfully weak (8)
OVERGROW (Jumble OR GROVE + W(eak))
40 Wag[n]er may depend on this Aussie reject (4)
DICE (Two meanings)
41 Rejecting [D]ad, sticking with oriental princess in myth (7)
HESIONE (ADHESION minus AD = E (Oriental))
43 More robust East European bit[e] (5)
HALER (Two meanings)
44 Work, [R]egency by the sound of it (5)
KNEAD (Homophone NEED)
45 South-east Asian holds can[e] from South America (6)
LATINO (TIN inside LAO)
46 Wi[l]de embraced by very small maid (6)
TWEENY (W(ide) inside TEENY)
47 Dee[m] to include such fruit (6)
DAMSON (SO inside DAMN )
48 Unclued (8)
MUSGRAVE
Down
1 Divers run, se[n]d flying head over heels (8)
SOMERSET (SOME (i.e. various = diverse) + R(un) + SET
2 Sages, eg, had constituted as such [seasonings] (8)
EGGHEADS (Anag. SAGES E.G. + 'D (had))
3 A place to be[a]t Irish artist (6)
RENOIR (RENO (Nevada, casino city) + IR(ish))
4 See gold more ador[n]ed in Scotland (4)
LOOR (LO + OR (gold))
5 Expression of surprise in co[d] fish (4)
COHO (OH inside CO)
6 Is desire to [b]eat the unconventional a spur (5)
SPICA ('S (is) + PICA)
7 Endlessly subject to a force to restore the [y]ears of the bard (6)
UNDEAF (UNDER minus last letter + A + F(orce))
8 State [impounding] possessions (4)
AVER (Two meanings)
9 Consider as perfect plan[e] lies wrecked (8)
IDEALISE (IDEA + jumble LIES)
10 Spiritless English tenor? I'll stick to rock [music] (6)
LIMPET (LIMP + E(nglish) + T(enor))
11 Hat worn [a]cock (5)
CAPON (CAP + ON)
13 Disease of cattle ra[n]ge needs to turn around (8)
WIRE-HEEL (IRE inside WHEEL)
17 Mollusc be[d] having a measurable effect on small space (6)
TELLEN (TELL + EN)
18 [B]assists on second Beatles record (5)
HELPS (HELP (Beatles album) + S(econd))
22 Very large European trailing Central Asian tin[y] creature (7)
MONGOOSE (MONGO (Mongolian currency) + OS + E(uropean))
24 They repair front of [motor] ending fine (6)
AMENDE (AMEND + first letter of ENDING)
"They" seems superfluous.
25 An Alvis i[s] getting reconditioned in plant (8)
SALVINIA (Anag. AN ALVIS I)
27 Greedily [he] eats swallowing bit of rock and humbugs (5)
PRIGS (First letter of ROCK inside PIGS)
29 Princess [a]top a lot of worms (8)
ANNELIDA (ANNE + LID + A)
30 Sure mo[n]k's developed a fragrant flower (8, two words)
MUSK ROSE (Anag. SURE MOKS)
33 Drank too much: thu[d]s gave pain as of old (6)
SOAKED (SO + AKED)
34 Rubbish is round about free [of] pleasures (6)
TREATS (RE (about) inside TAT + 'S)
36 Ship's chandler, a [w]retch on Queen Elizabeth (6)
ACATER (A + CAT + ER)
37 Take meal swallowing drug:compound [Q] (5)
DIENE (E inside DINE)
39 Bit of ribaldry I write to get re[a]d (5)
RIPEN (First letter of RIBALDRY + I + PEN)
41 Direct protection for pate[n] (4)
HELM (Two meanings)
42 Not [d]one with name unknown, socially unacceptable (4)
NON-U (NO + NU (name unknown))
43 Stop performing computation, [I] execute (4)
HANG (Two meanings)