Hello Robert,
I am a former Northants Officer and I have recently volunteered to work on the Force Heritage Project. I don't yet have access to their material but most of their documents are now curated by the Northamptonshire Records Office, which, like Police Headquarters, is at Wootton Hall Park.
A request was passed to me today with the link to this thread, I think because it contained contact details, now obsolete.
However, in recent months I have been independently researching the officers of the then three separate forces in the County, who served in WW1, one of which was Davey.
My working notes on him may contain useful information for you, below. Your information about the nature of his illness is new to me.
I wonder if my speculation about his war service is relevant. Between 1915-1919 the Berrywood Asylum was Northampton War Hospital. I know that some soldiers remained there, even beyond WW2, after it returned to being a mental hospital.
Hitherto I have failed to identify which Army unit PC Davey served in. He is one of about 6 from a total of 92 County Constabulary men who served, whose units have not been identified. I am visiting NRO in the near future in search of clues, but perhaps you know?
Anyway, my notes follow; please correct me if I have erred:
Charles Henry Davey was born on 2/5/1883 in Tasburgh, Norfolk, & was a carpenter before he joined the Northamptonshire County Constabulary on 3/2/1908, & in 1911 was a PC at Deanshanger, where he lodged. He married Beatrice Elizabeth Riches, his home village schoolteacher in 1912. They had a son, Jack, 1913-2001. Charles' military service was from 1/1917-2/1919 but his unit has not yet been identified. He was medically pensioned from the force on 2/2/1924 (impact of war service?) and in 1939 the family were living at Bedford Road, Northampton. Charles died in 1957 & Beatrice 3 years later.
All the best,
Dave