Who was the last Prime Minister of the United Kingdom to serve as leader of the House of Lords as well?

Question

Who was Prime Minister of the UK on 28 March 1979, when the House of Commons passed a Motion of No Confidence, forcing a general election that was won by Margaret Thatcher?

Answer 1

The Most Honourable Robert Gascoyne-Cecil, 3rd Marquess of Salisbury
The Marquess was Prime Minister, and leader of the House of Lords, 1885-6, 1886-1892, and 1895-1902. The role of the House of Lords in the Parliament was successively reduced through the 19th century, and no further Prime Ministers in the UK served their term wholly from the Lords..