Back to the drawing board
While sat in my office doing some cross referencing,I just happen to glance across the desk to where my latest copy of Grassbox (No 113) was sat,noticing that cracking photo that Clive Gravett has got in his collection.Then opening to Clive's writing " Every Picture Tells a Story".
I must say after reading the peice again,on Clive's detective work,well done in getting to find out where this picture was taken,by using google.
The only problem I have,is the mower on the picture is certainly not the mower in the 1910 catalogue, nor will it be for the proceeding catalogues to 1916.Yes Greens produced catalogues up to 1916,then ceased to 1919. Over lapping the Great War.Allowing that the 1919 catalogue was just a copy of pre war items.In 1920 Greens introduced the two cylinder engine for the 36 inch,which is the one in the photo.
Look closely at the picture,the mower is a 36 inch.The 42 inch machine did have this same type of engine fitted as early as 1908,but looked totally different due to the tank arrangement.Now going back to Clive's script about chauffeur's.Yes it is a known fact that you write about chauffeurs understanding the early combustion engine,but it also is a known fact that allot of those chauffeurs enlisted and never returned from the Great War,which is more than a probable reason to why your Groundsman is sat on that mower.
Why you are reading the School Year Books is beyond me,unless that is, if your Groundsman was a pupil! All I can remember from my school year books is something to do with Due Diligence
I only hope this helps you to find the real age of this picture.