What led British intelligence to believe that there was a highly-placed mole within their organization?

What are the problems involved in trying to find the highly-placed mole?

How does Smiley conduct the investigation? Why didn't he just work out of a government office?

What experience has Smiley had with Karla?

Who is Merlin and how does he figure in the investigation?

What is the significance of the term Tinker, Tailor?

How do they get the mole to reveal himself?