Question: Sir William Arthur Mount, 1st Baronet is the great-grandfather of which former British PM?
Context: Baron Rotherwick, of Tylney in the County of Southampton, is a title in the Peerage of the United Kingdom.  It was created on 8 June 1939 for the shipping magnate and Conservative Member of Parliament, Sir Herbert Cayzer, 1st Baronet.  He had previously represented Portsmouth South in the House of Commons.  Before his elevation to the peerage, he had been created a baronet, of Tylney in the County of Southampton, on 29 January 1924.  Cayzer was the fifth son of Sir Charles Cayzer, 1st Baronet, of Gartmore and the younger brother of Sir August Cayzer, 1st Baronet, of Roffey Park.  s of 2010 the titles are held by the first Baron's grandson, the third Baron, who succeeded his father in 1996.  He is one of the 92 elected hereditary peers that remain in the House of Lords after the passing of the House of Lords Act of 1999, and sits on the Conservative benches.  Lord Rotherwick also succeeded to the Cayzer Baronetcy of Gartmore on the death of Sir James Arthur Cayzer, 5th Baronet on 27 February 2012. Baron Gainford, of Headlam in the County of Durham, is a title in the Peerage of the United Kingdom.  It was created in 1917 for the Liberal politician Jack Pease, a member of the Darlington Pease family.  He notably served as President of the Board of Education from 1911 to 1915.  Pease was the second son of Sir Joseph W. Pease, 1st Baronet, and the grandson of Joseph Pease, while Arthur Pease was his uncle and Sir Arthur Francis Pease, 1st Baronet, Beaumont Pease, 1st Baron Wardington, and Herbert Pike Pease, 1st Baron Daryngton, were his first cousins.  The third baron was a former member of the London County Council and of the Greater London Council.  s of 2013 the title is held by his younger brother, the fourth baron, an architect and town planner; County Planning Officer for Ross and Cromarty 1967-1975 and Scottish Office Inquiry Reporter 1978-1993.  As a descendant of Sir Joseph W. Pease, 1st Baronet, he is also in remainder to this title. Baron Howick of Glendale, of Howick in the County of Northumberland, is a title in the Peerage of the United Kingdom.  It was created in 1960 for Sir Evelyn Baring, the former Governor of Kenya.  A member of the famous Baring family, he was the third and youngest son of Evelyn Baring, 1st Earl of Cromer, and the great-grandson of Sir Francis Baring, 1st Baronet, the founder of Barings Bank.  Baring's uncle was Edward Baring, 1st Baron Revelstoke, the father of Maurice Baring, while other members of the family include Francis Baring, 1st Baron Northbrook, and Alexander Baring, 1st Baron Ashburton.  s of 2014 the title is held by the first Baron's son, the second Baron, who succeeded in 1973.  As a descendant of the first Earl of Cromer and Sir Francis Baring, 1st Baronet, he is in remainder to both the earldom of Cromer and its subsidiary titles and the Baring baronetcy of Larkbeer, titles held by his kinsmen the Earl of Cromer and the Baron Northbrook respectively. The Dick baronetcy in Prestonfield, Edinburgh was created in the Baronetage of Nova Scotia for James Dick.  Initially created in 1677, it was renewed in 1707 and merged with the Cunningham of Lambrughton, Ayrshire baronetcy in 1829.  The family seat was Prestonfield House, Edinburgh.  Sir William Dick, 2nd Baronet and Sir Alexander Dick, 3rd Baronet were the younger sons of Sir William Cunningham, 2nd Baronet (of Lambrughton) and his wife Janet Dick, the daughter and heiress of Sir James Dick, 1st Baronet.  Both brothers changed their surname to Dick on inheriting Prestonfield in turn. Baron Harlech, of Harlech in the County of Merioneth, is a title in the Peerage of the United Kingdom.  It was created in 1876 for the Conservative politician John Ormsby-Gore, with remainder to his younger brother William.  He had previously represented Carnarvon and North Shropshire in the House of Commons.  Ormsby-Gore was the eldest son of William Ormsby-Gore, Member of Parliament for County Leitrim, Carnarvon and North Shropshire, and the great-great-great-grandson of William Gore, third and youngest son of Sir Arthur Gore, 1st Baronet, of Newtown, second son of Sir Paul Gore, 1st Baronet, of Magharabag, whose eldest son Paul was the grandfather of Arthur Gore, 1st Earl of Arran. Sir William Arthur Mount, 1st Baronet CBE (Hartley, Hampshire, 3 August 1866 – 8 December 1930) was a British Conservative Party politician and Member of Parliament for the Newbury constituency.  He is the great-grandfather of Conservative politician David Cameron, who was Prime Minister of the United Kingdom from 2010 to 2016. The Anson Baronetcy, of Birch Hall in the County of Lancaster, is a title in the Baronetage of the United Kingdom held by a branch of the Anson family.  It was created on 30 September 1831 for William Anson.  He was the third son of George Anson; his elder brothers were Thomas Anson, 1st Viscount Anson and General Sir George Anson.  Sir William was the uncle of Thomas Anson, 1st Earl of Lichfield and Major-General George Anson and the great-nephew of George Anson, 1st Baron Anson (see Earl of Lichfield, 1831 creation for more information on the Anson family).  His grandson, the third Baronet, was a lawyer and Liberal Unionist politician.  He never married and was succeeded by his nephew, the fourth Baronet.  He was the only son of Frederick Arthur Anson, third son of the second Baronet.  The fourth baronet drowned in the Thames in July 1914, aged only twenty-five.  He had not married and on his death the title passed to his first cousin, the fifth Baronet, the eldest son of Rear-Admiral Algernon Horatio Anson (1854–1913), fourth and youngest son of the second Baronet.  He was killed in action in the First World War.  He was unmarried and was succeeded by his younger brother, the sixth Baronet.  As of 2007 the title is held by the latter's son, the seventh Baronet, who succeeded in 1951.  He is a retired Rear-Admiral in the Royal Navy. The Dalrymple, later Dalrymple-Hamilton, later Hamilton-Dalrymple Baronetcy, of North Berwick in the County of Haddington, is a title in the Baronetage of Nova Scotia.  It was created on 29 April 1697 for the Hon. Hew Dalrymple, Lord President of the Court of Session under the judicial title of Lord North Berwick from 1698 to 1737.  He was the third son of James Dalrymple, 1st Viscount of Stair, and the brother of John Dalrymple, 1st Earl of Stair, Sir James Dalrymple, 1st Baronet, of Cranstoun, and Sir David Dalrymple, 1st Baronet, of Hailes.  The second Baronet sat as Member of Parliament for Haddington and Haddingtonshire.  The third Baronet represented Haddingtonshire in the House of Commons.  The fourth Baronet was Member of Parliament for Haddingtonshire, Ayrshire and Haddington.  He assumed the additional surname of Hamilton after that of Dalrymple.  The fifth Baronet sat as Member of Parliament for Haddington.  The eighth Baronet assumed the surname of Hamilton before that of Dalrymple.  The tenth Baronet was Lord-Lieutenant of East Lothian from 1987 to 2001. Philip Napier Miles JP DLitt "h.c." (Bristol) (21 January 1865 – 19 July 1935) was a prominent and wealthy citizen of Bristol, UK, who left his mark on the city, especially on what are now its western suburbs, through his musical and organisational abilities and through good works of various kinds.  He was the only son of Philip William Skynner Miles (1816–1881), a major promoter and developer of the docks at Avonmouth, who was the eldest son of Philip John Miles (1773–1845) by his second marriage to Clarissa Peach (1790–1868), and Pamela Adelaide Napier, daughter of the soldier (distinguished in the Peninsular War) and military historian General Sir William Francis Patrick Napier.  He was therefore half-nephew of Sir William Miles, 1st Baronet, half-cousin of Sir Philip John William Miles, 2nd Baronet, both Conservative politicians, and cousin of the fashionable portrait painter Frank Miles, gentleman cricketer Robert Miles and Mount Everest explorer, General The Hon Charles Granville Bruce.  He was educated at Harrow School and Oriel College, Oxford, and was selected as High Sheriff of Gloucestershire for 1916–17. David William Donald Cameron ( ; born 9 October 1966) is a British politician who was Prime Minister of the United Kingdom from 2010 to 2016 and Leader of the Conservative Party from 2005 to 2016.  He was Member of Parliament (MP) for Witney from 2001 to 2016.  Cameron identifies as a One-Nation Conservative, and has been associated with both economically liberal and socially liberal policies.
Answer: David William Donald Cameron