1) "The land of lost content" is taken from a poem by AE Housman.
Into my heart an air that kills
From yon far country blows:
What are those blue remembered hills,
What spires, what farms are those?
That is the land of lost content,
I see it shining plain,
The happy highways where I went
And cannot come again.
It is about the "good old days", when everything was so much better than now.... or that's how one remembers it. We were content then, but that has been lost. So, it means that many people (of my age, anyway) tend to think that Britain was a happier, more civilised country back in the 50's and 60's.
2) "Chaps" is an old-fashioned word for men of a certain background and outlook: upper middle class; probably educated at a public school, and with conservative views. These were the people who made up the Conservative Governments between 1951 and 1964. "Establishment Cliques" refers to the fact that many of the most important bodies which run the country (Political Parties, The Judiciary, The Church, the Armed Forces and the like) were actually controlled by a very small number of "chaps" who knew one another very well, and worked together to ensure that their positions were not threatened.
Remember that that's Andrew Marr's view, and it is not necessarily the whole picture. Cliques are not the preserve of any one political party. Some would say that Harold Wilson's Labour Government (which came to power in 1964), and the powerful Trade Unions of the time, were even more clique-ridden than the Conservatives.