With all the leads you have found Sally, I wonder if this is William and wife Louise and possibly sister Margaret in Hampstead in 1901. The visitor in the household looks interesting as although indexed as Haret, I think the original looks like Hurst. There is also the reference to Fermanagh for both William and Margaret:
William H Mitchell 48, British and Foreign....xxxx (lots of big black lines over the next words

) b. Fermanagh Ireland
Louis Mitchell 31, b. London
Leslie G Mitchell 2m b. London
Mollie E Mitchell Under 1 Month b. London
Margaret Haret 44, widow, b. Fermanagh Ireland
Eliza Whitley 38, staff
Charlotte J Howard 23, staff
Mable Martin 19, staff
Mary E Johncock 30, staff
Fanny E Morley 28, staff
RG13, Piece 120, Folio 126, Page 44.
Monica