It was one of those programmes that people in the trade vehemently denied had any similarity to actuality but confirmed in later decades that it was uncomfortably close to the truth. From what I knew of the MOD when I was in the RAF, it was definitely very close to the truth.
I see a number of people are now coming forward and claiming to be ex-spies from the 60s/70s. They reckon Tinker Tailor Soldier Spy was also a fair representation of the SIS, although they never had such raucous Christmas parties.