In the 1851 census Christian & Julia were in Ballaugh, as Frances wrote. You can find them under "Miller" (Christian Howard bapt 20 Jan 1789 Ramsey, daur of Thos HOWARD & Catharine CHRISTIAN).
Ballaugh, Ballakinnag:
Christian Miller Head Widow 61 Annuitant b.Ramsey
Julia [ditto] Daur U 23 Annuitant b. Castletown.
[So the "John Longford Mellor, Esq. Of Wrexham, Denbigh, North Wales" in the paper a few months later is strange, unless that is where he came from originally.]

In the 1861 census they are still in Ballaugh, Glen Tavern:
Christian Miller Head U 71 School Mistress b.Town Ramsey
Thomas P. Christian Son in Law Mar 35 Engineer b.London
Julia L. Christian Daur Mar 33 School Mistress b.IOM Castletown.

In 1871 Christian Mellor W[idow] 82 Annuitant b.Ramsey is one of two lodgers in Corvalley Ballaugh with Philip Shimmin and family (a farmer of 59 acres).

Brian Lawson's burials index has:
MILLER, Christian 87 Ballaugh 2 Apr 1876 [als] HOWARD.
There may be a MI for her, with other family members, for MELLOR, MELLER, or MILLER?
And/or for her husband John Longford Mellor who Frances wrote was buried in Malew graveyard on 15 June 1844. As John MELLAR according to Brian's index. Thanks, Donna, if you would check.

Sue