In "One, Two, Buckle My Shoe", 'Poirot' went to Somerset House on the Thames and spent the better part of a day in 1937 to search through the records for the wedding of his chief suspect.
According to Wikipedia, the Registrar General of Births, Marriages and Deaths set up his office in the North Wing of Somerset House in 1837. This established a connection that lasted for almost 150 years. This office held all Birth, Marriage and Death certificates in England and Wales. But if Poirot was around today, he wouldn't be looking up his information... information... information there - indexes to those documents are now at the Family Records Centre.
This fact may not be of much interest to Londoners who already knew about the place, but Anglophiles here in the U.S. might like to learn more about Somerset House.