Across:
   1. Some retro schoolboy care: mundane arithmetic etc. (8)
   5. Turn into a 14 perhaps, with enlargement (6)
  10. Field where things might be launched into orbit or stored in ice cap so ready to return (9)
  11. More sensible point put before one quarter start to rebel (5)
  12,13. Brother, it is what grows on scrotum when a young stud starts being a carrier! (7 7)
  14. Unpleasant finish on brie prepared in autumn (8)
  16. Space in which competition ultimately held? (5)
  18. Has a good look at how Lego's constructed (5)
  20. Ox is not getting leads... this is what reins are for (8)
  23,9dn. Cryptic clue shall read, e.g., "8 down 25 across" (7 2 6)
  25. Part with Rio travelling via this facility? (7)
  26. Tweak tail sections of outer fuselage and front parts of jet intake grate (5)
  27. Unaware of currently serving upper house having standing opening (9)
  28. Invest... at end having this! (6)
  29. Most inclined to omit loop with 20 nuisance (8)
Down:
   1. Like Neighbours, or Barney Rubble (6)
   2. Dean and Head of Immunology get together for a drink (7)
   3. Grandfather is kin, essentially shelter for danger (9)
   4. Conduct initially foolish, resulting in mishap (5)
   6. Let down one side, have debt to the other (5)
   7. Gate was broken, resulting in loss (7)
   8. Norm is one, an Île-de-France resident (8)
   9. see 23 across
  15. As specialised soldier, commit crime, being a paid killer (8)
  16. Carrier of a far nicer malady (3 6)
  17. Aircraft with nose section involving flexible attachment (8)
  19. Heads of lucrative empire's aerial roaming jaunts employ this (7)
  21. Bearing child is noble... gas consumed (7)
  22. Some lustre etched away? (6)
  24. Jolly fire (5)
  25. Thing with value as determined (5)