One Helen Graham birth found in 1967 but no helen E Graham births.  I think it may need their marriage certificate to begin to resolve this as this would give her age at marriage, and thus her birth year,  and her father’s name.  But not actioned yet.
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Their marriage index:

First name(s)       Alistair W
Last name           Smijth-Windham
Marriage quarter    3
Marriage year       1990
Registration month  8
Marriage Finder     Alistair W Smijth-Windham married one of these people
                    Helen E Graham
Spouse's last name  Graham
District            Wycombe
County              Buckinghamshire
Country             England
Volume              19
Page number         2102
Record set          England & Wales Marriages 1837-2005
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