"The Human Factor" - Who is This Small Man??

I’ve come to the end of Part Five of “The Human Factor.” Mr Halliday is the first surprise, and a far bigger surprise is “a small man with a black moustache wearing a dark suit carrying a black attaché case.” Who is this man?? After this man appeared, I was reading the last two pages of the part while hiding the remaining paragraphs with my hand. I want to know but I don’t want to know!! Great suspense! Greene never tells who in Part Five. And at the beginning of Part Six, he turns to Sarah and her mother-in-law. Who is the man?? Tell me but don’t tell me!!