Across:
   1. It may cause great loss of heart (10)
   6,16. 11 spot where 20 hit hard, because initial outlays ruined island (3 2 5)
   9. Good to be surrounded by level type of snake (5)
  10. Caught up in damaged net, tried to catch fish (9)
  11. Wanting new girl? Young lady is sipping a large water... (11)
  13. Disappointing score for vanilla core (3)
  14. see 21 across
  16. see 6 across
  19. Get rid of old country (7)
  20. Terrible 3 down harm came about (7)
  21,14. Learn, so this place might survive future 3 down (3 7)
  22. Pinching PM in meeting right out of order! (11)
  24. Great dominator of Turks relaxed an order (9)
  26. One pimple containing massive head and huge 1 across (5)
  27. ... --- ... ? (3)
  28. Rock afficianados to lose gigs with new arrangement (10)
Down:
   1. Coming from miserable Exeter, pom unprepared (9)
   2. Celebrates royal epochs (5)
   3. It may cause great loss, and wildly churn air before the end (9)
   4. French one at work? Yes please - rest not so good! (6 5)
   5. Flier falls short of strip (3)
   6. Being able to speak for more than one group? (9)
   7. Jump between registers during upcoming sale, do you? (5)
   8. Counter opens to accommodate next terrific influx (4)
  12. Street criminal immersed in toxic substance, which should get rid of the problem (4 7)
  15. Cow taken by wandering eels... do such things usually pose hazard to livestock? (9)
  17. Awkward fish swallowing moth (9)
  18. Watches made by Swiss group made exotic sounds, ultimately captivating gallery (9)
  21. Gets close to snare lunatic (5)
  22. I managed to be a neigbour to the Turks (4)
  23. Stone and Watson perhaps with same movements around start of May (5)
  25. If you don't finish what you're eating, you'll ultimately do this (3)