Some help in tracking down the Registrar's Offices for your post 1837 certificates
Buckinghamshire Registration Districts post 1837
The Register Office for each of the parishes in Bucks
National Registration Districts post 1837
As above but for the whole of England & Wales
National Reference Codes 1837 - 1946
National Reference Codes 1837 - 1974
If you find a reference on Free BMD or in the indexes the code will give you the district of registration
Registration Districts as of 2003
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Scottish Registration Districts
Download in either Excel or Adobe format