I was aware of the story - but not in the depth illustrated by Mark Felton here.
Makes no bones about the guilt of 'Uncle Joe' for personally ordering the murder of 12,000 Polish military POWs.
Allied POWs were brought in to witness the Nazi exhumations at the site near Smolensk.
Churchill believed the view of Polish government in exile, that the Soviets had been responsible.
But the Allies needed the Red Army to destroy Hitler.
Until 1989, Polish efforts to uncover the truth were supressed by the Soviet authorities.
No-one has ever stood trial for the Katyn massacres.
Bit of additional background here : Katyn Massacre | Polish history