SHORT of tumbrils and the Army taking over the TV station, it is hard to tell when a revolution has happened in a country like the UK with no single written constitution. But at the weekend, when I briefly and mistakenly heard that David Frost was to become the new Secretary of the Cabinet, I had an uncomfortable glimpse of something revolutionary in the air – and not in a good way. Its immediate threat may have gone, but some of the implications are still hanging.

First, Mr Frost as the National Security Adviser. This is a far less serious matter than his becoming Cabinet Secretary. It has few, if any, constitutional implications. I worked for a previous incumbent for a couple of years. It is an extremely important role but, as its name suggests, it is primarily advisory – and also a kind of personal envoy from the Prime Minister to counterparts overseas. That could be work for a trusted political appointee.

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Is it a good appointment? That is a more subjective question, and less fundamental. Mr Frost’s predecessors have been very senior diplomats, with deep grounding across Britain’s foreign policy and security concerns – former heads of the Diplomatic Service, Ambassadors in Washington, Paris, Permanent Representatives in Brussels, both to the EU and to Nato. Some have done more than one of these big jobs, in wide, senior international careers.

Mr Frost is able and, importantly, has the Prime Minister’s confidence. But his diplomatic career culminated in the important, but not central, ambassadorship to Denmark. He then ran the important, but not central, Scotch Whisky Association, and was special advisor to Mr Johnson, as a Foreign Secretary who earned mixed reviews. Judging Mr Frost’s role as chief Brexit negotiator may be premature. But it is odd, after the justified criticism of Sir Mark Sedwill’s being both Cabinet Secretary and National Security Adviser, that Mr Frost should have to combine the climax of Brexit negotiations with whatever Middle Eastern politics, for instance, or terrorism, may serve up in the coming months.

Choosing the new Cabinet Secretary matters more. Britain won’t acquire a written constitution any time soon. Much therefore depends on working through complex but broadly respected conventions. Over the last 100 years, as government has grown, so has the Cabinet Secretary’s role as a constitutional guardian. He – no “she” yet – advises the Prime Minister on constitutional issues and questions of propriety, as well as maintaining the good order of Cabinet business and co-ordinating departments. Often, though not always, he has been Head of the Civil Service too.

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There is therefore good reason for the common background of most Cabinet Secretaries to include formative service in the Treasury, and senior, repeated exposure to No10, often as Private Secretary to one or more Prime Ministers. They know about Parliament, the economy, cross-Whitehall management, and political and media realities. With a mainly diplomatic background, Mark Sedwill has lacked his predecessors’ grounding in our curious constitution: this must have been a challenge. Even if those appointing his successor want a revolution in Whitehall, it would be good if he or she thoroughly understands the constitutional arrangements to be overhauled, within which crises are bound to happen. Not least, in an increasingly scandal-prone world, the Cabinet Secretary should have a neutral authority to make judgments on erring ministers and officials that command respect across the political and public scene. Let’s hope these are the aims.

George Fergusson is a retired senior diplomat