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Using some of the info from the Guardian obit that Ros linked to ~
Her birth might be registered in the GRO Army Birth Indices. There is this one, which is a possible:
Catherine Macleod, Agra, Indian Subcontinent, 1914, page 217
These can be ordered via the GRO.
Her marriage to Ernest Gregson in 1998, Perth is listed on SP, so this can be ordered from that site. It should show her parents names.
Also, her sister's patronymic name is 'Màiri Oighrig nighean Choinnich 'ic Aonghais 'ic Choinnich
', suggesting that her father was Coinneach/Kenneth. (http://www.tobarandualchais.co.uk/en/person/5966
) Marietta MacLeod (m. Donald MacDonald) died in Morningside, Edinburgh in 1983 and, again, this is listed on SP and csn be obtained from that site, showing her parent's names.
SP = Scotlands People www.scotlandspeople.gov.uk
Hope this is of some help.