Across:
   1.  Novel trivia event aids the products of 27 (15)
   9.  Close bond made a racier mix (11)
  10.  Asian coal returned, a ton short (3)
  11.  It's related, or partly related, in reverse (4)
  12.  A sheep was found in Peter's settings (10)
  14.  Armchair golfer's first action (6)
  15.  Russian leader's dim rival butchered (8)
  18.  While playing flute, sat being inoffensive (8)
  19.  Lourdes lupin extract returns a type of power (6)
  21.  Avoid it with a plentiful 13 concerning request of many (10)
  23.  Squares of 10, 0 and 7 form spiral (4)
  26.  Asian principle: give thanks to nothing (3)
  27.  Fitting together reverse of 28 (11)
  28.  Picking apart reverse of 27 (15)
Down:
   1.  Turf turncoats! (5)
   2.  In places of worship, a model models (9)
   3.  Dash of retro jazz style (4)
   4.  Spoke like right one to make phone call (6)
   5.  Erratic and confused evil Arab (8)
   6.  Fantastic characters endure most (10)
   7.  Worth of veal lauded unevenly (5)
   8.  Ponds rose dramatically with support (9)
  13.  Range about, for each to anger (10)
  14.  Full of turds? A tea disaster! (9)
  16.  Dictator's irregular mission captured union and league leaders (9)
  17.  It informs you, but it's discrete about it (8)
  20.  Hot gunplay achieved nothing (6)
  22.  Quietly cover the mathematicians' goal (5)
  24.  Russian leader's fallen into partiality (5)
  25.  Some figures are a bit elevated (4)