Across:
8 Three from high street's ('three' from 'high streets' = ighsts = SIGHTS) places of interest (SIGHTS),
9 Barman (Senior Counsel = SC-) or (-OR-) detective (-PI-) gets to switch (-ON) (SCORPION) something with a sting in its tail (SCORPION),
10 Took public transport (bused) starting off (b) (‘bused’ without ‘b’ = USED) – the type that’s not new (USED),
11 Type of fluidity (ELASTICITY) in the Spanish (the ‘Spanish’ = EL-) sparking wine (-ASTI-) from Barcelona, for instance (-CITY) (ELASTICITY),
12 Miss Piggy (sow = SO W-) takes pork pie (pork pie = -HAT) (SO WHAT) – and why would that be significant? (SO WHAT?),
14 Reads the Riot Act in the house (LAWMAKER) to one of those trying to get rid of Old Bill (LAWMAKER),
15 Most of 24 across (most of ‘note’ = NOT) find area (A) with 19 down (‘pretty’ = PRETTY) and one of those in 8 across (one of those ‘sights’ = SIGHT) (NOT A PRETTY SIGHT) is as ugly as sin (NOT A PRETTY SIGHT),
18 Deep marl (deep marl) ground (= anagram indicator) (deep marl = RED MAPLE) found in the forest (RED MAPLE),
20 Sounds like alcohol that’s been diluted (WATERY) or whiskey (W-) from Bistro (eatery) that’s needs no introduction (e) (‘eatery’ without ‘e’ = ATERY) (WATERY),
22 Gold-digger (PROSPECTOR) in Coppers to (coppers to) dance (= anagram indicator) (coppers to = PROSPECTOR),
24 Write down (NOTE) what’s recalled (= reverse indicator) in native tongue (in ‘nativE TONgue’ = eton = NOTE),
25 One of those agitating for change (UPSETTER) provides leads for unusual person (leads for ‘Unusual Person’ = (UP-) with dog (-SETTER) (UPSETTER),
26 Complete (ENTIRE) set of good books (New Testament = -NT-) discovered in our place (Eire = -E-IRE) (ENTIRE).
Down:
1 Detailed (t) formal order (formal order/decree = 'fiat' without 't' = FIA-) for Glaswegian (Scot) doesn't end (t) ('Scot' without 't' = -SCO) (FIASCO) in disaster (FIASCO)
2 Get rid of (SHED) shareholder-owned banks (‘SHareholder-ownED’ banks = SHED),
3 Fan (= anagram indicator) resents a (resents a = ASSENTER) yes-man (ASSENTER),
4 A large part of the world (ASIA) caught up (= reverse indicator) in complaisance (caught up in ‘complAISAnce’ = aisa = ASIA),
5 Fawn (KOWTOW) King (K-) reacts to a sudden pain (-OW-) linked to pull (-TOW) (KOWTOW),
6 Expert (SPECIALIST) in A-list spice- (a list spice) -mix (= anagram indicator)? (a list spice = SPECIALIST),
7 Canines (DOGTEETH) of vampire tetras? (DOGTEETH),
13 In principle (principle), it’s said (= homophone indicator) (‘principle’ = ‘principal’ = HEADMASTER) it could be Freud? (psychoanalyst/head master? = HEADMASTER),
14 Fluid measure (LITRE) of some banana split reinvented (some ‘banana spLIT REinvented’ = LITRE),
16 It’s beyond mature (OVERRIPE) for Romeo (lover) to remove intro (l) (‘lover’ without ‘l’ = OVER-) on last letters (-RIP-) to sweetheart (‘swEet’ heart = -E) (OVERRIPE)
17 Of course (yes), for the most part (s) (‘yes’ without ‘s’ = YE-) coming with (-W) the ashes (TREES) (YEW TREES) traditionally found in graveyards? (YEW TREES),
19 It’s beautiful (PRETTY) in Nice (nice = PRETTY),
21 Undertakers asked out (‘undertakers’ ‘asked’ out = unertr = RETURN) – presumably, this is the reply? (RETURN),
23 Those on the road (TARS) or fronting The Abbey Road Sessions (fronting ‘The Abbey Road Sessions’ = TARS),
24 Catches (NETS) reviews (= reversal indicator) of characters in The West End (in ‘the weST ENd’ = sten = NETS).