Earlier this month, after the news was announced of Rohan Silva's depature from Downing Street, I listed some of those who had left previously:

• Tim Chatwin, head of strategic communications.

• James O'Shaughnessy, head of policy.

• Peter Campbell, who helped to prepare him for Prime Minister's Questions – as he did former Conservative leaders.

• Steve Hilton (who needs no introduction).

This morning, Rachel Sylvester, who has excellent Downing Street contacts (or so one must conclude from her columns), adds further names in the Times (paywalled link):

• Paul Kirby, another head of policy

• Kris Murrin, head of Downing Street's Implementation Unit, is on a year's sabbatical.

• Sean Worth, a senior adviser, and the man sent to the Department of Health to steady the ship over NHS reform. My apologies to Sean, who has strong views about public service reform, and should have been on my original list.

Rachel also writes: "I am told that of the 10 senior civil servants who originally worked in the Downing Street policy unit, only three now remain – and two of these are actively looking for new jobs."

Admittedly, personnel come and go from No 10 under any administration. But Cameron is experiencing a high attrition rate. Why?

The answer isn't hard to find. Non-civil servants who work for prime ministers are usually motivated by a) ideological zeal or b) personal ambition or c) both.

Cameron travels ideology-light. Consequently, those senior staffers driven by a particular ideal tend to leave – Hilton being the exemplar.

In little more than two years, Cameron may well be gone. Very few people in Downing Street – if any – are confident that he will still be in place in June 2015.

And (not to neglect the obvious) he leads a coalition, which means that his government has even more than the usual share of cobbled-up compromises.

The comparison with Margaret Thatcher, startlingly, suggests more similarities than differences – at least when it comes to senior staff turnover. Charles Moore refers to John Hoskyns, Lady Thatcher's first policy unit head, in Andrew Gimson's interview with him. Moore decribes a memo in which Hoskyns "absolutely rips into her about her leadership style. It's a rather brilliant encapsulation of her bad qualities." (The Spectator carried an interview with Hoskyns after Thatcher's death which is well worth reading.)

Hoskyns, who arrived with Thatcher in 1979, was gone from Downing Street by 1982 – a reminder that than even the most inspiring governments lose senior staff, often because they, like Hilton, believe that their advice is being flouted. The delightful Ferdinand Mount, whose memoir contains one of the funniest accounts of life with Thatcher ever written, also didn't last long in the post. John Redwood did two years before being selected to contest Wokingham.

Finally, Rachel's take on the will-he-won't-he departure of Andrew Cooper, No 10's director of strategy, is that he will return to Populus, the polling company he helped to found, after October. (So Peter Hoskin was right to be sceptical about Downing Street's denials.) But when is a departure not a departure? Cooper advised Cameron before his appointment, and he is likely to go on doing so after he leaves. He was always likely to return to Populus at some point, so I take suggestions of an unbridgeable gulf between him and Lynton Crosby with a pinch of salt.