Hi
Fleet marriages took place around the Fleet prison which was in Farringdon Street, on the eastern bank of the Fleet River (now an underground river). Fleet Street like Farringdon Street are in the area of the City of London.
Westminster is to the west of the City of London (now called the square mile). In 1746 this would be all London officially was. Westminster was in the county of Middlesex
http://en.wikipedia.org/wiki/WestminsterThe 1746 marriage on BMD registers is referenced to RG7/217 and 222 (Registrar General series at The National Archives)
which is the Registers of Clandestine Marriages and of Baptisms in the Fleet Prison, King's Bench Prison, the Mint and the May Fair Chapel
Because the Family Search site states the marriage took place in Westminster the likeliest of the places listed is May Fair Chapel in Westminster. As these marriages are held at The National Archives they are not part of the digitalisation programme Ancestry is undertaking with London Metropolitan Archives and the Guildhall Library to put online images of parish registers held by them.
A marriage pre 1754 usually holds little detail. e.g.
http://www.british-genealogy.com/parish-registers/marriage-registers-before-1754b.htmlIt is not until 1754 (with the law that ended clandestine marriages) that witness names were required.
The 1773 marriage was by licence. Peter Sandiford was a witness. When you say he consented to the marriage (because Elizabeth was a minor) do you mean you have a copy of the marriage licence which states he was the one who gave his consent to the marriage taking place? If so the licence should also give his relationship to the bride and her age. By tradition a marriage took place in the bride's parish but parish residency was only a three week requirement.
There is a baptism of a daughter of Peter Sandiford at Christchurch Southwark all in February 1757. His wife maybe called Sarah - the register is damaged and just the first two letters of her name appear. The child appears to be called ?arriot (probably Harriot), Rara?, Frances, Harland Jack?
Peter Sandiford is described as a gent - gentleman.
Burial
2nd January 1830 St Peter Walworth Southwark
Elizabeth Lloyd Russell aged 78, St Leonard Shoreditch
Regards
Valda