Who Dunnit?

S2, E11: Count Duckula has received a letter from a law firm in London saying that a relative (a Duke of Mallardborough) had died and left him (as well as others) something in his will. When Duckula makes it to London, the will reader tells Duckula and the other inheritors that since the Duke was murdered by a person within that room, that none may leave until one confesses.