The birth of Agatha Anne ALEXANDER was registered in Pewsey, Wiltshire, in Q1 1913. Her mother's maiden surname was SIMONS, and she was a scion of, but not descended from, the Earls of Caledon, being a 3 x greats-granddaughter of Robert ALEXANDER (1722 - 27 III 1790), an older brother of the first Earl. Following her divorce from Lt-Col Messel in 1945, she was married secondly, in 1946 (registered Q1 - Westminster) to Thomas Arthur RENSHAW (15 XI 1899 - 19 V 1984), whose mother was a daughter of the 4th Earl of Leitrim and granddaughter of the 2nd Earl of Leicester. She died on 23 March 2010.
It is clear that the late Mrs Allison's husband must have been born before 1948, in order to have been married in 1963 (from Free BMD, the Timothy J Allison whose birth was registered in Weymouth in Q4 1948 died as an infant, his death being registered there in the same Quarter).