This description is provided by Olive Thexton of Fylde Branch.

Marriage records begin with simply names of parties, then parishes of parties, witnesses and officiant, and whether after Banns or by Licence. They sometimes also have other information. 

From 1754 pre-printed register books were used, and from 1837 Register- Books supplied by Registrar-General were used.

Roman Catholic churches and non-conformist chapels needed the attendance at marriages of the local registrar and his book. They also may have continued their own marriage registers after 1837. 

Registration (other than of those who quietly lived together and passed themselves off as man and wife, perhaps because one had a previous partner still living) was universal. 

The problem came with the storing of copies centrally and indexing. The clergy were not always punctilious at sending to the Registrar General their quarterly returns. 

Churches had/have duplicate registers, one copy is sent on to the local registrar  when full. 

The quarterly returns are sent via the local registrar to the Registrar General. A marriage not found in the Government Record Office index is most probably an omission as all indexes are likely to have errors.