Theory:

     A bowler-hatted man near the mantlepiece greeted him as James entered his sitting room. “I am Johnson, sir. Security officer at Whitehall.” He showed his identity card and continued, “I presume, you know why I am here, sir.”
 
     “To the extent that is conveyed in this note,” replied James. He knew that it would be useless to ask Johnson for further details. “I won’t take long.”
 
     Johnson’s Ford Cortina brought them to Whitehall in less than ninety minutes. It took them another ten minutes to reach the chambers of Sir John Macpherson. Having introduced James to Sir John, the quiet but efficient Johnson slipped out.
Explanation:
 
A man with a bowler hat stood at the mantlepiece, as expected, waiting for James. He greeted James and took him to his room.
 
Place near Mantlepiece.jpg
Place near Mantlepiece
  
The reason was apparent; he had travelled all the way from Whitehall to accompany James, as stated in the letter. And the stranger was none other than a Whitehall security officer. Johnson was his name. Johnson showed his ID card and informed him of the reason for his visit. James told him that he already knew the information from the envelope he had received and that it would not take long to get ready to leave his room.
 
Even though James was curious to know the reason for the meet with Mr. Macpherson, he didn't want to ask question to Mr. Johnson since it would be useless. Because it was something official and confidential that Mr. John wanted to discuss with James. So, there would not be any possibility that Johnson might be knowing the details.
 
In Johnson's Ford Cortina, James arrived in Whitehall in less than \(90\) minutes. The Ford Cortina is a vintage car powered by a \(1.6\) Inline \(4\) engine with \(110\) bhp and a top speed of \(104\)mph. They arrived at Sir John Macpherson's chamber in about \(10\) minutes after then. Mr.Johnson departed after bringing James to John's room and introducing James to him.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Mantlepiece
A shelf made of wood, marble, or stone above and around a fireplace
Reference:
National Council of Educational Research and Training (2008). The Comet I - Jayant Narlikar (pp. 73-80). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.