I'm off down a rabbit hole now
I have found a Stuart Macdonald on the Medical Register for 1913 (there are two listed).
The one I am interested in is:
Name: Stuart Macdonald
Address: Army Medical Service
Date & Place of Registration: 1884, April 21 S
Qualifications: M.B. Mast. Surg. 1884, Univ. Aberd.
It seems he was born in 1861 in Elgin, Moray, Scotland to William Macdonald and Eliza Watson Stuart.
There is a paragraph on him in the Aberdeen Weekly Journal on 21st January 1916:
"Companion of the Order of St Michael and St George. Colonel Stewart Macdonald R.A.M.C - Graduated M.B., C.M, at Aberdeen University in 1884. He is the son of Mr William Macdonald, Elgin. He joined the medical service in the army in 1887, and was appointed a major of the R.A.M.C in 1899. He has the medal with two clasps for the Tirah expedition in 1897-98"However, it seems he died in 1939 as the Probate Death Index lists:.
MACDONALD C.B. C.M.G. A.M.S Stuart of Willowbank Moss-street Elgin died 19 May 1939 Confirmation of William Macdonald. Sealed London 26 August.He appears on the 1871 census as a visitor, in Paddington, London. Occupation 'Surgeon Army Med Staff Army Off', born Elgin, single.
1911 census - aged 49, single, boarder. Major RAM Corps at the Royal Army Medical College, Grosvenor Road Westminster. This is 1.4 miles away from where Bridget is recorded as living for Sheila's baptism in 1917.
I realise London is a big and busy place, however, it puts someone of the correct name, with the occupation that I have been told, in the same area.
There would be an age gap of approx 20 years between Bridget and himself.
Another tenuous link, when Sheila Macdonal married Hugh Sixsmith she married into a family of high ranking army medical officers. Surely those circles were small in those days?
Anyway... off to give it some more thought.
He could, of course, still entirely be a work of fiction!! And it doesn't bring me any closer to finding Sheila's birth, or who Bridget was. It's a puzzle.