ACROSS
1 Dullest dullard overwhelmed by exam (8) THICKEST {T{HICK}EST}
5 Fish by river, finding one gliding on surface (6) SKATER {SKATE}{R}
10 Entertaining literature: primarily, a deep source? (5) MILNE {MI{L
11 Hints end in sight amid unions' disruption (9) INNUENDOS {INNU{END}OS*}
12 Giving a name to mouse-pony hybrid (9) EPONYMOUS*
13 Year on island, topless resort (5) YALTA {Y}{
14 In conclusion, Guinness sells these! (6) STOUTS {
15 Impartial head of Ireland ignored in new EU test (7) NEUTRAL {N}{EU}{TR
18 Consume fruitcake with a bit of salad (4,3) PINE NUT {PINE}{NUT} Chilgoza in India
20 Stingy; losing pound will get you grief (6) MISERY MISER
22 Half of theological colleges regressive in Languedoc city (5) NIMES SEMIN
24 Having branches cut back to make room for using the net (3-6) NON-LINEAR {RAN}<= over {ON-LINE}
25 Arid broadcast from our station (5,4) RADIO FOUR {ARID*}{F}{OUR}
26 Primarily, text orthodox rabbi always honours? (5) TORAH Acrostic &lit
27 Father Christmas, catching cold, retreats (6) SANCTA {SAN{C}TA}
28 Drinks Australian ingested, displaying signs of overindulgence (8) PAUNCHES {P{A}UNCHES}
DOWN
1 Soften, harden (6) TEMPER [DD]
2 Telling off about part of Bible having been ripped off (3-6) ILL-GOTTEN {TELLING}* over {OT}
3 'Back off: yes? No?': Europe to UK, confused (4,4,4,3) KEEP YOUR NOSE OUT*
4 Impudence. Boy on Sabbath's clothing: comfy shoes (4-3) SLIP-ONS {S{LIP}ON}{S}
6 Observe horny suitor fumbling: 'Cool it!' (4,4,5,2) KEEP YOUR SHIRT ON {KEEP}{HORNY+SUITOR}*
7 Every so often, third yawl finds a kind of large wave (5) TIDAL {T
8 Naughty, vandalising cars with mate (8) RASCALLY {CARS*}{ALLY}
9 Workers' groups' stewards initially rising to show agreement (6) UNISON UNI(+s)SON(-s)
16 Energy lost from dodgy rear heater's element (4,5) RARE EARTH {R
17 'Happy, Sir?' (Man, unfortunately, is furious) (2,2,4) UP IN ARMS {UP}{SIR+MAN}*
19 Second-rate chump has to return prize (3-3) TIN-POT {NIT<=}{POT}
20 Endless acrimony with development for island (7) MINORCA ACRIMON
21 Hindu creator's not a creator of music (6) BRAHMS BRAHM
23 Following agreement, I'm reversing position (3-2) MID-ON {NOD}{IM}<=
Reference List
River = R, Year = Y, New = N, Pound = L, From = F, Cold = C, Australian =Boy = SON, Sabbath = S, Energy = E