Across:
   6. Such an equation has up to three roots coming up beyond its core roots (5)
   9,11. Artist has CGI & plastic model as devices to convey data (11 8)
  10. Traitor that is about to cross (5)
  11. see 9
  12,20. Conflict with Russia as a quarter in America upset with King (7 3)
  13. Doctor with work for each measuring device (7)
  14. Doctor's global medical group (3)
  15. Opposing it an issue? (4)
  17. Guys hang around centre of Heidelberg being fools (7)
  19. From the East, total energy and inspiration (4)
  20. see 12 across
  22. Mostly not gifted with the best of intentions (7)
  23. Tending to hurry back... bad sign! (7)
  26. Barrier around centre of pilloried Tuscan city (8)
  27. Cash's boy with a girl's name is up front as son perhaps (5)
  29. In our Darkest Hour, and in Stormy Weather, one that sang in Berkeley Square we hear? (11)
  30. Medicates animals with sulphur injection (5)
Down:
   1. Turn wise with practice (5)
   2. First half of initiation ritual heartlessly gruelling and dangerous (9)
   3. Humour hospitals taking elderly, but don't provide (8)
   4. Working on reserve cost Kipling badly (11)
   5,28. Kid's movie from bygone era, when everything was cool? (3 3)
   6. 150-in-1 collection initially a source of medical assistance (6)
   7. Willing brigade leader to get promotion (5)
   8. What a typical 19th century doctor needed to become, using monstrous lance that is held in two hands (9)
  12. Concerned with lust returned without love (in New York perhaps, which makes it easier to believe) (11)
  14. Initially wise old fellow who might care for the ladies? (9)
  16. Type of mechanics using modern measure in getting around one ... (9)
  18. ... part of car's controls repeatedly absorbing energy into cable (8)
  21. Possession of cervix enslaved angry women (6)
  24. Dangers when King is King's? (5)
  25. Getting up, starts to relish English weather, eagerly smoking pipe (5)
  28. see 5