I'm a novice so for my reference records could you please explain SMI and BIVRI and the scope in years the indexes cover.
SMI is The Sussex Marriage Index up to 1837, complied by the Sussex Family History Group. Contains most recorded Sussex marriages before 1837 but not all, no marriage recorded for your Elizabeth Ashdown. Could be that you have the wrong name here or a marriage outside Sussex or just not part of this index, perhaps there is no surviving record. Note that there are some possible marriages of Elizabeth's to Tree's, could you post up your evidence for Elizabeth Ashdown.
BIVRI is the British Isles Vital Records Index 2nd edition, a set of 16 Cd's produced by
www.familysearch.org, in most cases the data compliments the IGI but there is some overlap. Records mostly cover chistenings and some marriages, same date range as the IGI.