Many thanks to all for your interest and your finds.
wivenhoe, Florence died in retirement in Walton-on-Thames in Oct 1952 ; her (spinster) sister Emma lived next door. Long-lived and the last of their long-lived father's family: he was born in 1795.
amondg, thanks for finding and transcribing the advertisement. (**Is it acceptable to post paywalled links here?). I haven't found that advert anywhere else; e.g. not on BNA or Newspapers.com.
Lisa in California, I made the same assumptions: that the lawyers were performing due diligence in looking for family (I doubt that there was much of a will, but I haven't obtained it) and that Florence called herself Widowed for social convenience. We might find a death for Oliver in 1900-1910: but the lawyers didn't.
The Burgis & co. ad. is all the evidence I have that Oliver went to America. Not too good, I admit. He went somewhere, though! No sign of him in the UK records at all. Last record I have is 1891 census. I expect there are directories of Yarmouth, but none (?) online: when did the “Regent Toilet Club” close or change hands again?
amondg, Oliver Sumner landed 1904 in Canada, that's a find! I cannot see it, may I ask you for a link? Certainly Hugh Stephen Sumner who died in North York (Toronto) in 1944 is
my man's youngest brother : he was born in Peckham and emigrated in 1903 with his wife Florence and three children. Can we be certain that the
1904 farmer is Hugh's brother? That would clinch this, of course.