My grandfather's motorcyle (1920's)

[gsmonkey]

Hi

I'm a keen biker myself and was delighted to have found the pictures below of my grandparents out on their motorcycle in the 1920's (or early 30's??).  Having spoken to my dad, he seems to think that my grandad owned BSA's and Nortons.

Can anybody tell me what this bike is?  I know my modern machines, but am hopeless on the vintage stuff.  Please help, as I'd love to find out.

Thanks in advance.