Good sleuthng Christine. I would say that the Winifred Annie O'Sullivan (also rendered as Sullivan) born 1890 is a very good candidate.
And to continue the speculation (always allowed if one says it is speculation!), there were a number of births registered to the name Sullivan, mother's maiden name also Sullivan in the Romford District (which includes East Ham) in the 1916-1918 period.
Could well be the circumstances Diana posited initially.
Fascinating - an unknown English relative perhaps!!.
MaxD