Seems that I will have to let you out of your misery ( not that you will be miserable) and produce the answer.
The Horse (heroin) is "Under Water" - The Person is Len Deighton - The Agent is Harry Palmer (HP)
This is the Cottage where Len Deighton conceived and (apparently) wrote Horse Under Water. The novel is set in 1960, mostly in a small fishing village in Portugal (Albufeira), which was then a dictatorship led by António de Oliveira Salazar.
In common with several of Deighton's other early novels, the chapter headings have a "feature". In Horse Under Water these are crossword puzzle clues, reflecting the protagonist's habit of endlessly writing and replacing words in crossword puzzles.
The first edition of Horse Under Water published by Jonathan Cape was shorter than the later Penguin edition, which included a detailed description of the anonymous British agent's diving course, and also introduced characters later seen in the book, such as Chief Petty Officer Edwardes.
The plot centres on retrieving items from a Type XXI U-boat sunk off the Portuguese coast in the last days of World War II. Initially, the items are forged British and American currency, for financing a revolution in Portugal on the cheap. Later, it switches to heroin (the "Horse" of the title), and eventually it is revealed that the true interest is in the "Weiss list" — a list of Britons prepared to help the Third Reich set up a puppet government in Britain, should Germany prevail. Thrown into the mix is secret "ice melting" technology, which could be vital to the missile submarines then beginning to hide under the Arctic sea ice.