Thanks for your replies.
Looking to see if she was a widow, I searched for all BMD marriages for Florence with surname blank 1905-1916 in Newmarket, spouse name blank, couldn't find any to a Plumb except for the John Merry Plumb marriage in 1916.
I'd missed the CWGC ref to Florence Emma - so thanks for that. Soldiers Effects just says Florence E.
The birth in 1893 in Newmarket looks promising, as does the census in 1901 in Fordham, which came under Newmarket.
I agree the cemetery website for Lijssenthoek gives a lot of info you don't see elsewhere.
I found a death and probate record for a Florence Emma Plumb born 1893, also electoral register entries 1936-1938 for 31 Landrock Rd Crouch End (very close to Mill Hill where John Merry Plumb enlisted in the army).
I thought I'd found her. There is also a burial at Whitton, Richmon upon Thames at the same time, but the address is different.
Then...unfortunately, I also found a marriage for Florence E Plumb in 1922, registered Edmonton, Middlesex to a Frederick Percy Harrap, electoral register entries 1927-1939 at 10 Albion Avenue, Muswell Hill area (very close to Mill Hill where John Merry Plumb enlisted in the army), and a death in 1967 registered Barnet for Florence E Harrap, born 1893.
I changed the title of this post to include the name Harrap in case anyone sees it.
Jenny