sˈɪns æt lˈiːst nˈaɪntiːnhˈʌndɹɪd sˈɛvənti sˈɛvən, ɪt hɐdbɪn ˈeɪ "pɹəhˈɪbɪɾᵻd plˈeɪs" ˌʌndɚ sˈɛkʃən θɹˈiː bˈiː ʌvðɪ əfˈɪʃəl sˈiːkɹᵻts ˈækt nˈaɪntiːnhˈʌndɹɪd ɪlˈɛvən bɪkˈʌz ɪt hˈɛld klˈæsᵻfˌaɪd mətˈɪɹiəl fɚðə pˈɜːpəs ʌv fʊlfˈɪlɪŋ mˈoʊ dˈiː kˈɑːntɹækts.
Since at least 1977, it had been a "prohibited place" under Section 3(b) of the Official Secrets Act 1911 because it held classified material for the purpose of fulfilling MoD contracts.