Thanks for all the replies. I bit the bullet and rebooked the flight as one reward MIA-MAD-MAN so I would be assured to be on the Manchester flight. I did get the 25% Avios discount on the MIA-MAD sector but was gutted to lose that on the MAD-MAN sector for a flight that was selling for £39 including bag and seat selection. I will hold on to the original tickets and hope for a time change over the next 8 month.