I did a search in the Ancestry database: England & Wales, National Probate Calendar.
I searched for surname BLACK, passing on between 1970 -1980 in Salford. I got two hits. I would assume Gertrude's name would be in the probate record, as you said she received a considerable legacy.
6 Radford St, Salford (Note: name of street hard to read though)
died November 22, 1972
probate London July 20th
39 Murray St., Salford
died August 12, 1979
Administration Manchester 11 September
Hope this helps,