Asked to explain the British Constitution to university students, the Queen once said that it “has always been puzzling, and always will be”. This generation will be considerably more enlightened if they refer to the latest Hennessy blockbuster (at 500 pages) on our odd forms of governance.

This book could not be more opportune as we face the implications of the most sweeping constitutional changes in our modern history. Drawing on contacts in the ruling elite made when he was doyen of Whitehall correspondents, as well as his mastery of government files then, and since as professor of contemporary history at Queen Mary and Westfield College, University of London, Hennessy has given us an elegant, witty, insider’s account of the wielding of power in the United Kingdom.

But his affection for the traditional system is tempered by his worries over “the potentially over-mighty power of the premiership”. The danger has been implicit in the office of the monarch’s First Lord of the Treasury since Walpole, but custom and the collegiate tradition of decisions taken in Cabinet have tended to rein in any premier with a dictatorial streak (wartime conditions excepted). There are now, Hennessy feels, disturbing signs of the biggest centralisation of power seen in Whitehall in peacetime - opening, in the words of one of his top civil servant contacts, “a back door into an American-style presidency”.

And, the author warns, the trend may be irreversible. He refers scathingly to the Modernising Government White Paper as “a scattering of eye-catching pledges replete with new public management argot and Billy Graham-style testimonies of various breakthroughs on the customer-care front. The whole production smacked of a mixture of Pollyanna and piety which caused in me an almost involuntary recoil.”

In another reference to the Blairite tendency, Hennessy says: “For someone who is altering the political map of Britain irreversibly, Blair shows no interest in constitutional matters.” He observes that in nearly three yearsas Prime Minister, Mr Blair has not made a single speech on the programme which has been the subject of at least 20 major Acts or Bills since 1997.

Contrast this with the author’s nostalgia for the Attlee years, which he sees as the high point of Cabinet government. This unassuming, underrated man brought into office the accumulated experience of understudying Churchill, as well as leading his party, in a variety of specialist roles in the most trying wartime conditions. His quiet sense of purpose in Cabinet kept a restive and sometimes mercurial team focused on a common goal - the creation of a welfare state. Hennessy cites a delightful example of Attlee’s naturalness. A 15-year-old schoolgirl wrote him a poem complaining:

“Would you please explain, dear Clement, just why it has to be that certificates of education are barred to such as me?

I’ve worked through 13 papers but my swot is all in vain because at this time next year I must do them all again...”

To which the Prime Minister replies (marked Secret):

“I’ve not the least idea why They have this curious rule Condemning you to sit and sigh Another year at school You’ll understand that my excuse For lack of detailed knowledge Is that school certs were not in use When I attended college.”

The author reminds us that it was another underrated premier, Jim Callaghan, who in 1976 first floated the idea of a core curriculum as a beacon in educational reform, only to see it “borrowed” three years later by the government of the most controversial of former education secretaries, Margaret Thatcher.

Hennessy, a “would-be rescuer of her reputation”, is ultimately defeated by her arrogance, quoting that moment of exasperation when she implored after a Cabinet meeting: “Why won’t they do what I want them to?” Tony Benn claimed that prime ministers who are remembered are those who think and teach, and Mrs Thatcher influenced the thinking of a generation. But as the author and Douglas Hurd, her former foreign secretary, agree, her singular lack of skill in putting across her case, in this case to her cabinet colleagues, led ultimately to her downfall.