Saddle tank, hand gearchange, inverted levers, girder forks- deffo a pre-war bike,- but that hardly narrows the field as we had a booming bike industry back then. No civilian bikes built after about 1941-apart from low powered autocycles for essential users. Nice pic-shame there aren't any more showing side view of machine but I guess the photographer was trying to capture your gran rather than the bike! At a guess Id say 1930s as flat tanks were mostly gone by then & electric lighting had replaced acetylene lamps. Do you know whether your Gran rode much?