Twelve solutions (not further defined) belong to one of two groups (six apples and six pears), which as ‘apples and pears’ are cockney rhyming slang for a flight of stairs ie a LONDON FLIGHT (23,25 across).

The definition part in each of the remaining 16 clues contains a single letter misprint. In clue order, these incorrect letters spell out two locations for another sort of 23,25 across: HEATHROW and STANSTED.

Across

1 bergamot GREAT MOB (anag) [pear]

5 hobble (H/b)OBBLE [skirt/sHirt]

9 fastener FAST + RENE (rev) [tie/tEe]

10 Idared I DARED [apple]

11 inexpert (n)INE/X/PERT [inept/inApt]

12 codlin CODLIN(g) [apple]

14 odious foOD 10 USually (hidden) [nasty/Tasty]

15 pasta PAST/(mafi)A [food/Hood]

18 eater EA<TE<R [apple]

19 tee off T off PART = PAR/normal [hole/Role]

23,25 London flight

24 arboreta A/R(right)/BORE/TA [elm/elO]

26 animated A MAN I (anag) + TED [moving/moWing]

27 Comice EC(on)OMIC (anag) [pear]

28 Pearmain (th)E/ARM in PAIN [apple]

Down

1 biffin BIFF/IN [apple]

2 russet R(ugby)/U(nion)’S on SET [apple]

3 Aleppo APPLE (anag) + O(rders) [fighting/Sighting]

4 overridden RIDDEN after OVER (6 deliveries) [weary/Teary]

6 Old Norse L(50)/RED/SOON (anag) [former/fArmer]

7 Bartlett BAR/T<LET<T [pear]

8 endanger END + (skatin)G in NEAR (anag) [risk/riNk]

13 Conference CON/F<ER<ENCE [pear]

15 prolific PRO/L(earner)/IF/1/C(ollege) [lots/loSs]

16 scandium C(arbon)/AND/I(odine) in SUM [rare/raTe]

17 atrophic PORT (rev) in CHINA less N(orthing) (anag) [wasting/Easting]

20 Colmar MOLAR/C(anine) (anag) [pear]

21 Bertha BERTH/A [name/Dame]

22 warden ’ARD in W(est) EN(d) [pear]