I have only read one spy novel in my life but understand
that in addition to intelligence (D-INT) there is also a counter-
intelligence function in MI6. I never heard of a D-CINT. Did I
miss something? Perhaps Burnside took this job after the conference.
MI-5 and MI-6 are the domestic and foreign intelligence agencies
respectively (I hope I've got that the right way around) corresponding
to the FBI and the CIA in America. (The FBI also has other,
non-intelligence-related activities, of course.) The actual name for
the domestic agency in Britain is the British Security Service.
The operation of which Burnside is a part (headed by C) is neither MI-5
nor MI-6. The characters frequently refer to 5 and 6, often in derisive
terms. They also worry about stepping on 5 and 6's toes... remember when
Willy was nosing around in Gibraltar, and someone came to him in his
hotel room and announced himself as "So-and-so, Security Service"? He was
pissed off that Willy was on his turf (Gibraltar being under British
control, it qualified as domestic.)
Neil's agency is in fact the SIS, the Special Intelligence Service, and
is a department of the Ministry of Defence.
D-INT (Edward Tyler) was not Director of Intelligence for MI-6; he was
Director of Intelligence for SIS. SIS did not make a distinction between
foreign and domestic operations. Neil was D-OPS, director of Operations;
and as such the Sandbaggers were only part of his responsibility. He
actually ran all SIS operations. The Sandbaggers were his problem-solving
squad within SIS, who got sent in when things were going wrong.
Hope this clears things up. I think the series developers just found it
easier to dream up a completely new agency with its own hierarchy than to
try to weld something onto the real ones; besides, in Britain you're not
really supposed to talk about the real ones (whereas in America, the CIA
has its own Web pages!).