A Lucy MacKinnon aged 72 died in Birmingham in the first quarter of 1969. If your Lucy's birth was later than that in 1896, that death would be a perfect match for her. Possibly a child was living in Birmingham at the time and she moved after her husband's death.
Husband William died in 1953 in Kettering reg dist. There are two female McKinnon marriages in that reg dist in the right time period.
Isabella Y MacKinnon married in 1943; Isabella Young Tucker, born in 1921, died in 1998 in Watford. (That name seems quite Scottish; scotlandspeople confirms the birth of Isabella Young MacKinnon in 1921 but credits are needed to view details ... but it was apparently not in Dalziel.)
The second marriage was in 1954 so too recent to post details (also a typically Scottish female given name). There is one birth showing to that marriage. The couple were on the electoral roll in Corby after 2005. The child can be found on line.
seahall seems to have been correct about Angus's marriage and child -- have you got that info now?
If you look for births in Kettering with surname MacKinnon, there is another set in the 1960s (with mother's surname starting with D). There is no marriage to match that surname - possibly it was a second marriage for the wife.
There are also births in Kettering in the 1950s
- with surname starting with W and mother's surname MacKinnon, with no marriage to match
- with surname starting with A and mother's surname MacKinnon, and no marriage to match
- with surname starting with D and mother's surname MacKinnon, marriage 1951
Do you know whether daughter Mary had a middle name? That could certainly help identify a marriage and/or death.
There are 2 MacKinnon marriages in Salisbury in recent years that might be worth looking into. One can be ruled out by the birth and parents' marriage info, but I don't see a birth for the other. There is also a MacKinnon marriage there a generation earlier, but again I don't see a birth for that person.
Probably the most useful thing would be to get the death certificates for Angus and William, and also the certificate for the Lucy MacKinnon death noted above, to confirm whether it was your Lucy. If the informant of death was a family member in any of those cases, it would give you their name and address at the time of the death.