You may have found this information in the interim, but England Select Marriages (ancestry) has: Thomas MITCHELL, father given as John MITCHELL, marrying Ann Matilda STEVENS, father given as Samuel STEVENS, 13th April, 1839, at Heathfield, East Sussex
FreeBMD reference: Volume 7, page 439, 'Hailsham' district, poss St Mary's, Warbleton, East Sussex