This is revealed in an article headlined Quiet Sinners written by Bernard Porter in the current edition of London Review of Books (Volume 35 Number 6). The book concerned is Empire of Secrets: British Intelligence in the Cold War and the Twilight of Empire by Calder Walton (Harper, London £25.)
Discussing MI 5’s habit of bugging places where men and women of importance meet and discuss their affairs (political, that is) the reviewer says: “One of these, as Ian Smith suspected during the Rhodesia talks of 1965, was Lancaster House, where many of the negotiations over self-government took place (it was thought the décor would impress the natives). Smith used to meet his aides in the ladies’ lavatory, convinced that MI 5 would be too gentlemanly to plant mikes there.”
I suggest this is a scene that might one day be included in a Bond film, though I doubt Her Majesty (who is known as a good sport when it comes to the film antics of 007) would allow one of Buckingham Palace’s lavatories (aka toilets) to be used for anything other than the literal convenience of visiting dignitaries. – Trevor Grundy, UK