Thanks for the replies.
I know he did some sort of trials on his bikes, including some on the Isle of Man, there are some small trophies somewhere. This is the only other picture I have of the Ariel.

This is my father's bike, reg NHA 386, not showing on the DVLA site so may no longer exist. I would love to know what type of Ariel it is and what he was doing. Shirt and tie and no helmet! I'm guessing the photo is late 40s or early 50s. He later had an AJS 500 twin which is still on the road but n...