The header photo for this blog shows a group of old men in swimming costumes and top hats on (I also used it as a background photo for a Powerpoint presentation when I did the talk on toppers at the NSC in March 2012).
I had thought it was mid-Victorian based on the style of toppers worn and it seems I was correct based on this website:
The photo is of Brighton Swimming Club, dated to around 1863 and taken by Benjamin Botham.
The fascinating thing is (halfway down the page) the discussion on dating the photograph by examining the style of top hats and costumes the men were wearing; from stovepipes to bell crowns. This is a good guide to use to date hats of that period.