There are two John Santer birth records from 1937 to 1957 in England and Wales; one was born in St Pancras in 1945 with a mother’s maiden name of Page.  The other was born in Bristol, also in 1945, with a mother’s maiden name of Barlow.  I would incline to the former as he and Rachel settled in the St Pancras area for the births of their first two children.  But this reasoning cannot have a great certainty.

Obtaining his marriage certificate would give his birth year and his father’s name.  Then obtaining one birth certificate would enable a comparison with the father’s name on the marriage certificate.  But this would be a heavy £22 and the truth may emerge from other records for less cash.
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Their marriage index:

First name(s)       Rachel B
Last name           Smith-Windham
Marriage quarter    4
Marriage year       1970
Marriage Finder     Rachel B Smith-Windham married one of these people
                    John Santer
Spouse's last name  Santer
District            Westminster
County              London
Country             England
Volume              5E
Page number         1495
Record set          England & Wales Marriages 1837-2005
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