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English Heritage (officially the English Heritage Trust) is a charity that manages over 400 historic monuments, buildings and places. These include prehistoric sites, medieval castles, Roman forts, and country houses.

What is now generally known as the blue plaque scheme was initiated in 1866 by the Society of Arts.  In 1901 it was taken over by the LCC and then in 1965 by its successor body, the GLC. When that was abolished in 1986 English Heritage took on the scheme. 

Over the years hundreds of plaques have been erected and sadly some of these have been lost, often when their host buildings have been demolished.  Steve Roffey has given himself the task of identifying these lost plaques and listing them on Wikipedia.  Documenting existing plaques is a big enough task but finding the lost ones - that's far more difficult - for obvious reasons. Chapeau.