Across:
8 Come to terms with (FATHOM) the budgetary excess (FAT-) on unfinished (e) accommodation (home) ('home' without 'e' = -HOM) (FATHOM)
9 Challenges (DISPUTES) for 15 across opening (opening of ‘dispute the facts’ = DISPUTE-) school (-S) (DISPUTES),
10 Take a few courses (DINE) popular (-IN-) inside Germany (.de = D-E) (DINE),
11 Boxers (pets) backed (= reverse indicator) (pets = STEP-) by relative (-FATHER) (STEPFATHER) in The Guardian (STEPFATHER),
12 Faith (CHURCH) has short (l) fling (‘hurl’ without ‘l’ = -HUR-) with head of college (head of ‘College’ = -C-) in Switzerland (C-H) (CHURCH),
14 Puts a stop to (SILENCES) delicatessens throwing out dates (‘delicatessens’ throwing out ‘dates’ = elicsens = SILENCES),
15 Take issue with (DISPUTE THE FACTS) Barney (barney = DISPUTE) contacting heartless (i) thief (thief) (‘thIef’ without ‘i’ = THE F-) – the bookish type following John? (Acts follows John in The Bible = -ACTS) (DISPUTE THE FACTS),
18 A (A-) starter served (starter ‘Served’ = -S-) with a side of turkey (a side of Turkey = -SYRIA-) that’s top-notch (top ‘Notch’ = -N) (ASSYRIAN) produced by old Iraqi (ASSYRIAN),
20 Moral values (IDEALS) of ladies’ (ladies) movement (= anagram indicator) (ladies = IDEALS),
22 Was involved in (IMPLICATED) medical tip (medical tip)? Doctor! (= anagram indicator) (medical tip = IMPLICATED),
24 Foremost calories (foremost ‘Calories’ = -C-) in butter? (fat = FA-T) (FACT) Every scientist knows that! (FACT),
25 Detectives (detectives = DIs = DIS-) work out (-AGREE) (DISAGREE) how to cross Swords (cross swords = DISAGREE),
26 Strikingly unconventional (EXOTIC) engineering (E-) associated with toxic (toxic) mixture (= anagram indicator) (toxic = -XOTIC) (EXOTIC).
Down:
1 Victor (V-) has a (-A-) broken (= anagram indicator) shin? (shin = -NISH) (VANISH) Would you go away out of that! (VANISH),
2 Where the old woman lived (old woman lived in a SHOE) in Oxford (Oxford SHOE),
3 A huge success (SMASH HIT) for Ashley (-ASH) “Rocky” (= anagram indicator) Smith (smith = SM- HIT) outside (SMASH HIT),
4 Even mild sleep (even letters of ‘mIlD sLeEp’= IDLE) is good for nothing? (IDLE),
5 Some House fulminations (some ‘hoUSE FULminations’ = USEFUL) can serve a purpose (USEFUL),
6 The very latest arrival, for example (JUST IN), on trial (CASE) (JUST IN CASE) as a precaution (JUST IN CASE),
7 Passionate (VEHEMENT) strange (= anagram indicator) event (event = VE-ENT) over The Border (-HEM-) (VEHEMENT),
13 Presumably, someone will lose interest if you do this? (REPAY A LOAN) You might have to do it to keep a shark happy! (REPAY A LOAN),
14 Underworld figure (SATAN) similar to 17 down (SATAN similar to ‘evildoer’),
16 Will definitely stop you going out for the night (INSOMNIA) in this state – not even in your wildest dreams! (won’t have ‘dreams’ with INSOMNIA),
17 Doesn’t do anyone any good (EVILDOER) producing for bad actor? (EVILDOER),
19 Detective (DI) does a twirl (= reverse indicator) (DI = ID) in front of (= position indicator) Number 10 (CARD) (ID CARD) – presumably, it’s revealing who he really is? (ID CARD),
21 A late-night session (LOCK-IN) in most (s) bars (bars/bolts = locks) (‘locks’ without ‘s’ = LOCK -) where drink is served (inn) endlessly (n) (‘inn’ without ‘n’ = -IN) (LOCK-IN),
23 The odd characters at Rovers (the odd characters/letters ‘aT rOvErS’ = TOES) and those on 24 down, for instance (TOES on ‘foot’),
24 Soft option to ignore points (‘soft option’ to ignore ‘points’ = ofto = FOOT) produced by one of those down at heel (FOOT).