sentences:  He remained an influential critic and potential leadership challenger, and would eventually prove instrumental in Thatcher's fall in 1990.Brittan was then forced to resign for having, earlier that month and with the agreement of Thatcher's press adviser Bernard Ingham, ordered the leak of a confidential legal letter critical of Heseltine.For a time Thatcher's survival as Prime Minister seemed in doubt, but her personal involvement in the leak remained unproven and she survived after a poor debating performance in the Commons (27 January) by Opposition Leader Neil Kinnock.