Thank you everyone for your replies, they all give me something to think on.
I maybe wrong about William Colin being the child in the photo, I do know he was a private during WWI in the 16th and 9th Battalions of the Notts & Derby Regiment. He would have been 18 if he's the child but if he's the one in the uniform then the photo would have been taken sometime from 1914 to 1920. In that case the child is not his because he was still single in the 1939 register!
Surely you wouldn't be going to all the trouble of dressing up & travelling to a studio/photo booth for a four generation photo when the child wasn't yours and the older man is not family, it doesn't make sense.
So far I haven't been able to find out if Wm Colin's father William Gregory was in the military around 1897, this isn't my family so I need to wait on more information. I'll be back when I have something.
Thank you all again, it is much appreciated.
Cheers....Dawn