It wasn’t allowed to appear on Ordnance Survey maps until 1995.

From The Road to Little Dribbling: Adventures of an American in Britain by Bill Bryson. Page 103.

“We parted the next morning, but there is a postscript to our adventure in the woods. The hotel we stayed at in Lyndhurst was called the Crown Manor House Hotel. It seemed a decent enough place to us all – not hugely friendly or charming or well run, but decent enough – but soon after our visit Andrew forwarded to each of us an interesting article from the Southern Daily Echo of Southampton, concerning the hotel’s devotion to hygiene. The article stated:

A Hampshire hotel has been ordered to pay more than £20,000 in fines and costs after preparing food in rat-infested areas. The Crown Manor House Hotel in Lyndhurst, which twice closed its kitchens after inspectors found evidence of the infestation, admitted five food hygiene offences in a case heard at Southampton Magistrates’ Court. They included two offences involving the “production, processing and distribution of food in areas where there was ‘an ongoing infestation of rats’.

‘I thought those peppercorns tasted funny,’ I quipped merrily, but I was genuinely astounded to read about this, and for two reasons. First, I was naturally a touch chagrined, as you might expect, to learn that I had been staying in a hotel that was so slyly squalid, but I was also, and almost equally, amazed to find that I could now read about this sort of thing in a daily newspaper. I worked for the Southern Daily Echo’s sister paper in Bournemouth for two years in the 1970s, and in that time I don’t believe we ever ran a story about a filthy hotel or restaurant. That wasn’t because there weren’t filthy hotels and restaurants, I am sure, but because those things were secret.

Everything was secret in Britain then. Everything. People’s lives were secret. They “hid their houses behind tall hedges and put net curtains in the windows so that no one could see in. Almost everything the government did was secret. There was even a law, the Official Secrets Act, designed to make sure that essentially no one could know anything. It was quite extraordinary when I think back on it. Among matters that were classified in Britain in those days, ostensibly on grounds of national security, were: levels of chemical additives in foods, hypothermia rates among the elderly, the carbon monoxide levels of cigarettes, leukaemia rates near nuclear power stations, certain road accident statistics, even some proposals to widen roads. In fact, according to the wording of Section 2 of the Act, all government information was secret until the government declared it otherwise.

Sometimes all this became a little ridiculous. During the Cold War, Britain had a programme of building rockets for the delivery of warheads, and naturally it needed to test them. It was notionally a top secret programme. It even had a slick secret code name: Black Knight. The problem is that Britain is small and doesn’t have vast deserts in which to conduct secret tests. In fact, there isn’t “any part of Britain that is really secret at all. For various reasons, it was determined that the best place to test the rockets was at a famous landmark and popular tourist site on the Isle of Wight called the Needles. The Needles are clearly visible from the British mainland, so the firing up of the rockets could be seen and heard for miles around. A friend of mine told me that whole communities used to turn out on the beaches of southern Hampshire to watch the smoke and flames. Even though the firings were visible to thousands, the tests were officially secret. No newspaper could report them. No official could speak of them.

Even better was the Post Office Tower in London. For over a decade and a half, it was the tallest building in Europe. It dominated the London skyline. Yet because it was used for satellite communications, its existence was officially a secret. It wasn’t allowed to appear on Ordnance Survey maps until 1995.

Hadn't thought about the Officials Secrets Act in years. Bryson is right, it was commonly cited in articles and conversation and in TV shows. Everyone took it for granted.