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This dossier is almost certainly in the same place that all the paperwork that was seized from Police officers investigating Cyril Smith is kept. That investigation was raided by, and stopped by the security forces, because Smith knew where all the bodies were buried. MI5 acts on orders from above, so the corruption over these historical investigations runs to the very top of the Establishment.
Investigations such as these have been thwarted for many years, for the same reason, and it is still happening today. When Simon Danczuk said that he was going to bring up the name of Leon Brittan in front of the Commons Select Committee earlier this summer, he was jostled in the Palace of Westminster by some senior Tory figures, who tried to put the frighteners on him. He was warned that if he mentioned Brittain's name, that there was a risk that Brittan would commit suicide. Needless to say, he ignored that threat and Brittan is still alive today.
Danczuk's efforts to pursue the investigations into Smith is still being frustrated now, as stories have been circulated about him benefitting from the royalties from his book "Smile for the Camera-The secret life of Cyril Smith"