And I would of thought more like the 40s/50s judging by the lampposts etc?
Here's more food for thought, notice carefully the bright crankcase part next to your fathers left leg and note there could be a circular raised bit on it. Now compare with this 1933 New Imperial"https://www.google.co.nz/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjioOie5t_OAhUEp5QKHelgB3QQjRwIBw&url=http%3A%2F%2Fwww.roncobb.com%2Fnewimperial.html&psig=AFQjCNGEkXYT9iwBUW49nsJcx4a_ywS_uw&ust=1472325463720356Bit of a long link from Google but it seems to work