Across:
8 Position (position/post = OFFICE) of rotten (OFF-) spire at the centre of (‘spIre’ at the centre of = -I-) church (-CE) (OFFICE),
9 Similar to one of those opening 15 across (similar to one of those opening ‘Dutch elm disease’ = Dutch = EUROPEAN) for union member? (EUROPEAN Union member),
10 Flag (small flag on a boat = JACK) on top of 13 down (on top of ‘Jack Horner’ = JACK),
11 Initial (initial) problem (= anagram indicator) (initial = IL-INATI) hangs over run-down neighbourhood (slum) dismissing leader (s) (‘slum’ without ‘s’ = -LUM-) (ILLUMINATI) – a subject of conspiracy theorists (ILLUMINATI),
12 Content of Metro – January’s (content of ‘meTRO – JANuary’s’ = TROJAN) Man with reputation for hard work (TROJAN),
14 Epidemiologist’s study (DISEASES) is at the end of 15 across (at the end of ‘Dutch Elm disease’ = DISEASE-) at School (-S) (DISEASES),
15 From Holland (DUTCH) to the Spanish (the = EL-) mainland, foremost (mainland, foremost -M) detectives (Dis = DIS-) with no difficulty (-EASE) (DUTCH ELM DISEASE) produced a threat to those in 24 across (DUTCH ELM DISEASE is a threat to ‘elms’),
18 Where you’d hope to get fruit (ORCHARDS) or (OR-) beets (beets = -CHARDS) (ORCHARDS),
20 Problem for the electrician (short circuiting = FUSING) as he is not into Feng Shui (‘he’ is not into ‘Feng Shui’ = fngsui = FUSING),
22 Drew an analogy between (CORRELATED) officer (CO-) and head of regiment (head of ‘Regiment’ = -R-) from the same family (-RELATED) (CORRELATED),
24 They provide shade (ELMS) for one similarly in 23 down (ELM- ‘tree’) with skinhead (‘Skin’ head = -S) (ELMS),
25 Outlaw (RENEGADE) angered (angered = RENEGAD-) tipsy (= anagram indicator) engineer (-E) (RENEGADE),
26 & 13 down Corner boy at Christmas getting plums (LITTLE JACK HORNER) from a figure celebrated in a nursery rhyme (LITTLE JACK HORNER).
Down:
1 Involvement (AFFAIR) in a (A-) party (Fianna Fáil = -FF-) atmosphere (-AIR) (AFFAIR),
2 Folk music lacks focus (‘folk music’ lacks ‘focus’ = lkmi = MILK) to exploit (MILK),
3 Refuse to obey (DEFIANCE) prospective partner (fiancé = -FIANCE) after (= position indicator) short (n) study (den) (‘den’ without ‘n’= DE-) (DEFIANCE),
4 Old magazine (BELL) taken up (= reverse indicator) by Lucille Ball (taken by ‘luciLLE Ball’ = lleb = BELL),
5 Pleasant smells (AROMAS) from a (A-) chipper (-ROMA-) at top of the Strand (top of the ‘Strand’ = -S) (AROMAS),
6 They’re used by sailors (SPINNAKERS) and cricketers (bowlers/spinners = SPINN-ERS) over a (-A-) weekend (‘weeK’ end = -K-) (SPINNAKERS),
7 Mate’s (mates = PARTNERS) parents (parents) working (= anagram indicator) (parents = PARTNE-S) around river (-R-) (PARTNERS),
13 See 26 across
14 Tips (DUMPS) for Frenchman (-M-) consumed by baked (= anagram indicator) puds (puds = DU-PS) (DUMPS),
16 Thrown out of The Garden (UPROOTED) by awful (= anagram indicator) prude too (prude too = UPROOTED),
17 Popular (IN-) revolutionary (-FIDEL- Castro) society (-S) (INFIDELS) with non-believers (INFIDELS),
19 Horrible (= anagram indicator) ordeal (ordeal = RELOAD) to get stock in again (RELOAD),
21 It’s quick-moving (NIMBLE) for 1000 (Roman 1000 = -M-) bass (-B-) in river (River Nile = NI-LE) (NIMBLE),
23 Some reindeer treat (some ‘reindEER Treat’ = eert) dug up (= reverse indicator) (eert = TREE) in the forest (TREE),
24 Devours (EATS) CD-ROM released by Democrats (‘CD-ROM’ released by ‘Democrats’ = eats = EATS).