I hope they add something to what has previously been posted, but apologies in advance if I’ve missed a post which makes this one redundant!
Peerage.com shows she may have died 23 March 2016, so aged c105 at death.
To confirm the above (although still shown as living DPB online]: Cicely Abigail MILNE d 2016 GRO reg Q1 Richmond upon Thames, birth year 1911 & Ancestry reg as Lady Cicely Abigail MILNE d 31 March 2016, Middlesex [her son the 3rd Baron's DPB2019 address is Teddington, Middx]. She was d of Ronald LESLIE 1877-[1967] of Buenos Aires, Argentina, by his 1906 m reg Q3 Hants to Marjorie Sybil HALL 1880-1973. She m 1940 the 2nd Baron MILNE 1909-2005 and had 2 sons (incl present 3rd Baron) and a dau.