Hi Vesta
Sybil died before Dec 1936 ...
The Times, Wednesday, Dec 09, 1936; pg. 17; Issue 47553; col B
Fortcoming Marriages
...
MR H C S TREVANION and MISS STACEY
The engagement is announced between Hugh Charles Seaton, only
son of Mr. C.C. Trevanion, of 12, Vicarage Gardens, Kensington,
and the late Mrs. (Sybil) Trevanion, and Mary Hope, elder daughter
of Mrs. Stacey of Rottingdean, Sussex, and the late Major G A
Stacey D S O
...
The use of parantheses "(Sybil)" suggests that Charles had already remarried (to Florence) otherwise the item would have caused confusion about the identity of "the late Mrs Trevanion".
Hugh was born 20 May 1912 (from his GRO Deaths Index entry).
His step-mother Florence was born 12 November 1891 (from her GRO Deaths entry). In view of her age, we can assume that Charles married her sometime after 1921.
So ... Sybil's death should have been registered between 1921 and 1936.
If we knew when Joanna was born, this would help narrow the search (for the second marriage) and therefore the year of Sybil's death.