Sir Keir Starmer is the 43^rd longest serving prime minister in our country’s history. If a brave enough soul challenges Andy Burnham to become the next Labour leader, Sir Keir could still leapfrog The Earl of Aberdeen – a mid-19^th century, landowning Peelite – into 42^nd place. 

Assuming a coronation or the very possible failure of another candidate to get the support of the 81 MPs needed to stand, however, Sir Keir will stay 43^rd. He will have been in Downing Street for just two years and a few days in change. That’s one place and a few months ahead of Sir Anthony Eden, whose premiership was considered extremely short when he quit on grounds of ill health in 1957, shortly after the Suez crisis. 

We’ve only had 58 prime ministers, yet five of the past six rank well outside the top 30 for length of time in office. Even David Cameron, who now appears to be a beacon of longevity at a little more than six years, fails to crack the top 20. 

There is little doubt that each of those failures of durability can be explained by the unique circumstances of the time. This is also, however, a clear pattern that cannot be ignored by either politicians plotting their routes to the top or a public affairs industry that relies on a degree of stability to co-ordinate campaigns. 

“Political gravity, accepting conventions and the inevitability of quitting under pressure are concepts rather than …