Before 1754 many marriages were not performed in parish churces due to the need for people to get around the need for banns and other petty regulations. The favourite alternative venue was the Fleet Prison which was outside the jurisdiction of the Church of England. It is though that around 230,000 marriages were celebrated thre by the 1740's before legislation was tightened up by Hardwicke's Marriage Act 1754 (which said that weddings had to take place in an Anglican church).
Fleet marriages were heavily frowned upon by the Church, though were valid in law. The practice was banned with the introduction of Hardwicke's Act 1754, which required a license to be granted or banns to be called, although some ministers did continue to practice them illegally for a time.
The Fleet Registers collection held at the National Archives (called RG7) has now been made available by S&N Genealogy Supplies on their websites www.bmdregisters.co.uk and www.thegenealogist.co.uk.
Related Website: http://www.thegenealogist.co.uk