Hello Alan O
He didn't join up until 1919 (he was born in 1902). I have a copy of his service book entitled 'Regular Army Certificate of Service' inside it says 3rd Hussars and describes him as a trooper in the Corps of the Cavalry. I've attached a few photo's, another one from WW2 and then two between the wars.
I've just spoken to my dad whose found an old army cap that belonged to my gf. It has Military Police and GR on it, on the inside it says 'J Compton & Sons 1943'
Thanks, Phil