Miles M.18 Flight Characteristics
Peter Amos, the Miles Aircraft Historian, has answered most of the questions regarding the spinning characteristics of the M.18 in his book:
Volume 1 of ‘Miles Aircraft – The Early Years’ by Peter Amos.
Chapter 31
Miles M.18 Trainer Mk.I, Mk.II, Mk.III and Mk.IV HL
An extract from a test flight report quoted in the book mentions that it was impossible to spin the M.18 with the flaps extended. However, the aircraft did spin or, perhaps, spin/spiral, with the flaps up.