I looked at the 1861 census and Susan Garrett is "Gt. Grand Aunt" unmarried age 86 born Warrington. That makes her born about 1775.

Assuming this relationship is correct, which is probably true because it's the same in 1851, she will be a sister of one of Robert Henry Whitford's grandparents. We already know that RHW's maternal grandparents are Robert Jones and Jane Shimmin. Susan is unmarried, so it's unlikely she belongs to either of these (unless she's a half-sister).

That leads to her belonging to the paternal grandparents, who are unknown. You said previously that Roberts' father was Richard Whitford born ca 1785 place unknown. I bet that his mother was a Garrett, and perhaps older than Susan. She might also have been born in Warrington, but probably with IOM connections. She may have been born in IOM.

Whether she belongs to the Elizabeth Garrett who married Paul Gelling in 1802 is anybody's guess. Paul was born 1778 so probably Eliz is about the same. Last child was bp 1820. Paul drowned in 1834 and is in Braddan Old Yard South #484, as Donna recently said. I don't think Eliz made the 1841 census, so my choice for her (caution - this might not be right) is the burial of Elizabeth Gelling age 58 April 11 1834 Braddan. This is 6 months prior to Paul. It's possible that the small stone #485 with the inscription worn away is her, but of course it could be somebody else within the larger Gelling family in the group of MIs. I know nothing more about Elizabeth Garrett.

Having written the above, it occurred to me that the great aunt might be to Robert Henry's wife Ellen Brown, though I think it's less likely.

Ellen was born about 1819 Liverpool.
I think this is her on IGI: Ellen Brown bp 18 Apr 1819 St. Peter Liverpool, d of William Brown & Elizabeth.

Looking at 1841 for Robert Whitford & family, in addition to his immediate family he also has:
Ellen Daniels 84 Yes
Susan Garratt 64 Yes
William Brown 60 Mariner S
Elizabeth Brown 53 Yes

The latter two are likely Ellen Whitford's parents. They are in 1851 census at Toxteth Park. I had it in this note but accidentally deleted it, but from memory I think he was 78 Mariner born Scotland and she was 65 born Liverpool. Didn't find them in 1861.

Ellen Daniels age 84 could be interesting. She likely died before 1851. There's an Ellen Daniels death in Sep 1846 quarter Liverpool. There's one in Sep 1847 for Warrington, and a few others elsewhere.

Hope this helps a little bit.