Foxhunt by Bertie Denham
Derek Thyrde, a junior Whip in the House of Lords, discovers a Cabinet Minister is a blackmailer and is planning to take over the government by violent means. Together with Camberley Roberts, who tells him she is a police officer, he sets out to show his fellow politicians the truth. The author, Lord Denham, is the Government Chief Whip in the House of Lords.