The Mayfair Chapel was a popular venue for clandestine or irregular marriages in London before Lord Hardwicke's Marriage Act came into effect in 1754. These were marriages performed without banns or licence, and without parental consent in many cases. They were perfectly legal and valid, even if the Church did not like them. The Mayfair Chapel took a lot of marriage business away from the nearby parish church of St George Hanover Square.
cCandestine marriages are often known collectively as Fleet marriages, after the most popular venue for them, the area around the Fleet Prison, but they were performed in a variety of locations around London. They were particularly popular with runaways and others for whom secrecy was important, but most people just wanted a quick, no-fuss ceremony.
From your point of view, Pennie, the good news is that of all the clandestine marriage venues, the Mayfair Chapel was much the classiest, favoured by the aristocracy.
Mean_genie