SLORMGORM kicks off the week…
Good solid Monday morning puzzle from this reliable setter. A few things were new to me (mountain and bird in 20a and 16d), but gettable nonetheless.
Thanks SLORMGORM!

ACROSS
1. Book tough footballer (8)
HARDBACK
HARD (tough) + BACK (footballer)
5. A table on the Titanic? (6)
ABOARD
10. A mariner caught in storm (7)
ASSAULT
"a salt" = ASSAULT (a mariner, "caught")
11. Truthfulness is really not fruitful in the end (7)
HONESTY
HONEST[l]Y (really, not [fruitfu]L (in the end))
12. One cuts people off in late on-air broadcast (9)
ALIENATOR
(LATE ON AIR)* (*broadcast)
13. Old lady giving away million is something else (5)
OTHER
[m]OTHER (old lady, giving away M (million))
15. Henry & Son Labourers (5)
HANDS
H (Henry) + AND (&) +S (son)
16. Serve as soldier… one into his guns will do this, we hear (4,4)
BEAR ARMS
"bare arms" = BEAR ARMS (one into his guns will do this, "we hear")
19. What I’d do for The Platter’s costume party is also what The Drifters might do! (2,6)
GO ASTRAY
GO AS TRAY (what I'd do for the platter's costume party)
20. Mountain discovered in planetology (5)
ANETO
[pl]ANETO[logy] (discovered in)
21. Vacuous heads will detain everyone student digs (5)
HALLS
H[ead]S will detain ALL (everyone)
23. Chilled criminal robs banks ultimately without issue (9)
CHILDLESS
(CHILLED)* (*criminal) + [rob]S [bank]S (ultimately)
25. They cover crash sites or nuptials where bishop knocks out wife (7)
BEDDING
WEDDING (nuptials, where B (bishop) knocks off W (wife))
27. Seed I got near Aldi to invigorate anaemic starters (7)
IGNATIA
I G[]ot] N[ear] A[ldi] T[o] I[nvigorate] A[naemic] (starters)
28. Dutch film about grasping local is in the can (6)
TINNED
(D (Dutch) + ET (film))< (<about) grasping INN (local)
29. Old country’s colonial name is awful, so I heard (8)
RHODESIA
(SO I HEARD)* (*is awful)
DOWN
1. Principal with long excuse to avoid relations? (8)
HEADACHE
HEAD (principal) with ACHE (long)
2. Full-of-themself type changing dollars in US (7,4)
RUSSIAN DOLL
(DOLLARS IN US)* (*changing)
3. American prudes seen in titillating Hooters (9)
BLUENOSES
BLUE (titillating) + NOSES (hooters)
4. Manage clubs with initially unmanageable winger (3,2)
CUT IT
C (clubs) with U[nmanageable] (initially) + TIT (winger)
6. Game point coming up? Got to avoid last place! (5)
BINGO
(NIB)< (point, <coming up) + GO[t] (to avoid last place)
7. Dope found in service gets male excommunicated (3)
ASS
[m]ASS (service, gets M (male) excommunicated)
8. One in utility room seen with doctor inhaling grass (5)
DRYER
DR (doctor) inhaling RYE (grass)
9. Army chap turned drug dealer (8)
PHARMACY
14. Old man in costume with hero on the move (4,7)
HOMO ERECTUS
(COSTUME with HERO)* (*on the move)
16. Noddy Holder? (8)
BIRDCAGE
Cryptic definition, noddy being a kind of bird
17. Wild group performing with XTC in commercial (9)
ABANDONED
(BAND (group) + ON (performing) with E (XTC, ecstasy)) in AD (commercial)
18. Component used in robot swan approaching land (8)
BOTSWANA
[ro]BOT SWAN A[pproaching] (component used in)
21. Sailor in the grip of heroin and sex addiction (5)
HABIT
AB (sailor) in the grip of (H (heroin) and IT (sex))
22. Standard request for people about to be shot? (5)
SMILE
Cryptic definition, as in having a photograph taken
24. Teacher one upset with bit of harsh language (5)
IRISH
(SIR (teacher) + I (one))< (<upset) with H[arsh] (bit of)
26. Mostly accomplished academic (3)
DON
DON[e] (accomplished, mostly)