Across:
   8. Oddly "headier" hippie movement celebrated such growth (4)
   9,4dn. Fellow with trunk smoked green... grounds for one of the 8 numbers? (10, 7)
  10. Relating movement is essential (8)
  11. Provider of grass which is cut became a downer (6)
  12. see 18dn
  13. Break down cooked snail, say (8)
  15. Consumes his drug - a number from 8 (7)
  17,19dn. Hippie movement bar yoghurt perhaps? (7 7)
  20. A square shed exterior, one with America's number in 8 (8)
  21. Performed as cooperative "tribe" explicitly dismissing leaders (5)
  24. Major scale production for booze (6)
  25. Extra voltage essential for musical star (8)
  26. Shop staff sell rigs as assembled (10)
  27. Human, perhaps, not good as an 8 number (2-2)
Down:
   1. Returned some mail on Gambian vegetation (8)
   2. Musical sounding land (6)
   3. Sting returning trolleys (5)
   4. see 9ac
   5. Clean home in order for apparently adaptable fellow (9)
   6. Professional who used 15 in pursuing beasts (8)
   7. X-Factor's got no information for singers (6)
  14. Goal: story reworking for stars' assistance (9)
  16. Rears one surrounded by suspicions (8)
  18,12ac. Steel crucible forged for number out of 8 (8 5)
  19. see 17ac
  20. Places to play are not available by the second (6)
  22. Singer croons by not working (6)
  23. He played with the devil, and was quick having his turn (5)