There should be one allen screw facing down at the top of the seatpost, under the saddle where the rails of the saddle meet the seatpost. Once loosened, it should swivel back and forth - this is the same screw you adjust for forward/backward adjustment of the saddle (where the rails of the saddle are held in). Loosen and adjust to your liking. I have a Raleigh M50, assuming the M20 is the same.