Its a 500cc 'Sloper' Ariel from the early 1930's. Hard to find nowadays but one any Ariel fancier would give his eye teeth for.
The Ariel sloper range came and went very quickly and I suspect this was solely because of the dreadful economic conditions of the time. The depths of the Great Depression was not the time to introduce a new range of machines. Like everyone else Ariel had to make and sell only those bikes they were absolutely sure the public would buy. There was no room for error. The new Sqare Four stayed but the sloper range didn't.
Thanks for letting us see such a happy family photo.
Cheers,