Across:
   4,24dn. 16 cult's event has age shaken (6'1 4)
   8. Playing harps hid suffering (8)
  10. Bit of vodka one's allowed as help when "leaving one's vehicle"? (5)
  11. What 4 24 aimed for: multiple "5"s, each seeing one escaping being bound by natural limits (4 5)
  14. Security and Intelligence leads have self-obsession as focus of 4 24 (3 5)
  15. 75% of altruism gets diverted into largely symbolic activity (6)
  17. Con 16's mob contrived wins members to "5", as 4 24 suggested a spaceship was following this (5 4-4)
  19. Science-fiction fun's turned into murders (6)
  21. Turned earth in garden plot and got some oxygen in (8)
  22. Windows (higher version) offends, allowing entry with empty key (9)
  25. Brought back cocoa with classic espresso & foam - finally a nice drink!
  27. Ring alright if protected by thugs who keep watch (8)
  28. Some opposed ace detective returning long time product of 7 worn by 4 24 during 5 (7)
Down:
   1. Islander crazy, accepting silver as tin (10)
   2. P.S. McFadden dumped in the centre (8)
   3. Doctor 26 cycled (3)
   5. Opening for evacuation in latex item (4)
   6. TV lies about most evil (6)
   7. Cult's favourite saying "Just do it!" at the heart of The Kinks' rise (4)
   9. Having, in catastrophe, no bar bit ale supply, which proved fatal for 4 24 (13)
  12. Initially expression of humour (3)
  13. Tragic results of the sad nonsense? (6)
  15. Some free fertiliser made from grass clippings and paper (6)
  16. Arranging a pit, we help organiser of mass 13 (10)
  18. Conventional old transporter coming second to new hot rod (8)
  19. Initially expression of distress (3)
  20. Crucial points to be obesrved in harmful craziness (6)
  23. Cattle trashed 7 (4)
  24. see 4 across
  26. The way 3 cycled (3)