Grid by Ben Robinson and clues by Steve Allen

Across:
   8. Newton: “I model in reverse. Revealing this, one can have a knock-on effect” (6)
   9. Internet nasty? Yes, nasty.  These will take the load off (8)
  10. A quantity of beer in Germany for a clever German (8)
  11. To be angry, voir le Anglais! (6)
  12. A unit of refined sugar’s not right (5)
  14. Too ugly. At beautification, improves or doesn’t improve, maybe (9)
  16. Two accounts? One, as they show at the florists (7)
  18. Spaceman's nut, fish (7)
  20. If icon “ABC ...” forms a sequence (9)
  21. Put down the Top Ten perhaps  (5)
  24. The English language’s understood. Generally universal primary language  (6)
  25. Bird that can’t fly? Fly is unfortunately a way to get pulped (8)
  27. Internally yet are neglected, returning to procreate (8)
  28. I clued cryptically for ‘mathematician’ (6)
Down:
   1. Or it makes doughnuts maybe? (4)
   2. These might be the last things you write (6)
   3. Ego delivers ... delivers logic, man (5)
   4. Great fan’s unknown – as poet’s work (7)
   5. Plant story, as discussed, Rusty said at first (9)
   6. In cell, short bugle played for singers (4 4)
   7. Greek character? Sounds like a Greek character at the endless Greek gathering places (10)
  13. Damaged ears picked up, rang principled guy (10)
  15. Hardy colleague works most of January, man (9)
  17. Most of slow cures produce results (8)
  19. Mensches, no ... no crazy ideas (7)
  22. Bisexual, not half! Court and split (6)
  23. Mathematician’s result – i.e. not 1st – incredible (5)
  26. Curses, pilfered! First four returned (4)