Recent discovery

I thought I'd found all of my father's notes and geneaology documentation but while looking through some crates and boxes in the garage, I came across an old briefcase of his, a file of family research notes and this photograph.

My grandfather, Charles Roberton is stood at the back in the middle (the one with a cigarette in his mouth). I assume this photograph was of the team of park keepers at Balloch. There's no date or writing on the photograph so I don't know for definite but the architecture of the building in the background looks very similar to Balloch Castle (pictured below).