Great photo of someones' mum !
With tank between the frame rails, it (the bike) belongs firmly in the flat tank era - late 1920s or earlier.
First inclination is to say its a JAP sidevalve, v-twin obviously, and something like a New Hudson or New Imperial. With what seem to be fircones over the valves, would make it a sports model - and the high exhaust and low handlebars add to this impression of a late 20s sports model.
Couple of things about the engine look a little different though, will leave it for others to comment. Valve springs are hard to discern there ?