I am not sure if I am able to give the information being requested, because of the various posts on 'Sibby' Cooper, including one post regarding the possibility of him being a gardener for Mr Murphy.
I am the remaining sibling of the said Mr Richard Murphy, and unfortunately my sister who passed away recently might have had the information you are requiring. My late brother would have also had this information, but alas, this post as arrived rather late for them to offer any help.
My own understanding of a 'Sibby' Cooper, was either as a tenant farmer or manager of a small acreage farm at Copt Oak, Leicestershire, which my father owned. Whilst in this roll of farmer, 'Sibby' was badly crushed by a cow. The farm was also reduced in size, due to the M1 motorway dividing the property, when motorways were the new idea for faster transportation. So the actual farm became non-viable.
As I understand 'Sibby' then may have taken up part-time gardening activities for my father.I also recall, that 'Sibby' was a pro boxer, as were many people at that time, making spare money. I also seem to recall, that part of Leicestershire, like most rural places, the Markfield area, was a tightly knit community, with each family having some form of tie with each other. Mining in the area, was also a very big employer, with quite a number of well known coal mining pits.
'Bobby' Copson was the very well known local resident police officer, so perhaps a delve into any police records held at that time, might bring further directions for added information, on 'Sibby' Cooper in the 1930/40/50 era.
