Suggestion -- The structure on the photograph puts me in mind of the largely wooden "theatres" on part of the cattle market that received shows and circuses in late Victorian times- it could be Duval's from the 1870s or Mortimer's from the 1890s. George Sante famously developed the latter with boxes and lighting before it burned down in 1914. The gathering looks like a performance of HMS Pinafore or a concert for the navy. The dress would place it early twentieth century.