Hi Alison,
This is not a perfect look-up I'm afraid - still too many potential "spanners in works". The following may help you. There are no Mary Millers marrying any Michell/Mitchell but you need to remember that the index only goes up to 1837 (1839 for a few places). But the Charles looks a reasonable bet.
John MILLER m Ann ELLIS, botp (B)
Lewes St. John sub Castro, East Sussex, 13 Feb 1825:
John MILLAR, Maresfield m Lucy WILMSHURST
Buxted, East Sussex, 22 Jul 1822
John MILLER, bach m Mary ATTREE, sp botp (B)
Brighton, East Sussex, 12 Jul 1819
John MILLER m Elizabeth REED, botp
Maresfield, East Sussex, 7 Feb 1829
Charles MILLER m Caroline WENHAM
Westmeston, East Sussex, 20 May 1832
Good luck
Nell