You have to say right away that Steve Richards is very fair to politicians. It is an admirably unfashionable habit among political commentators. Some scribblers nowadays would concoct an affair between David Attenborough and the Queen if either secular saint were to show an inclination to vote Labour.

All the same, a writer can go too far. In his ten essays on prime ministers we never had, Richards devotes thirty-five pages, more than to anyone else, to Jeremy Corbyn, including to his policies (much of them mainstream social democratic stuff, Richards claims), his leadership (sic) and strategy (or lack thereof), as well as to the ‘appallingly’ disloyal behaviour of those on the right of the Labour Party who helped to destroy him. Richards’s tone is generous, which makes his conscientious cataloguing of Corbyn’s shortcomings all the more devastating. Jezza never really wanted the job, explains Richards. You don’t say!

Yet Corbyn was not alone in feeling ambivalent about becoming PM. Richards rightly stresses how often impostor syndrome has stalked the corridors of power and crippled the campaigns of even those who were more ruthlessly single-minded (though not necessarily more talented) than Corbyn. Rab Butler and Roy Jenkins both wanted the top job, but not enough to kill for it. Barbara Castle (the book’s only woman) was flattered to be tipped. But she never trimmed or plotted to get there ahead of Margaret Thatcher, arguably her inferior in brains, energy and courage, though – at that stage – much less abrasive.

Denis Healey and Ken Clarke (who tried three times to be Tory leader) were battle-hardened and hugely experienced ministers, and manifestly papabile. Neither, however, would deign to stoop low enough to conquer the lumpen backbenchers or ‘Silly Billy’ activists whose support they needed. Both were also loyal. Furthermore, Clarke and two other of Richards’s figures, Michael Heseltine and David Miliband, were hobbled by their Europeanism. In some ways, Clarke was actually a tougher Thatcherite than Thatcher. But the Eurozealots didn’t notice that. Heseltine had all the qualities needed to be PM, including looks and hunger, but colleagues did not trust his politics or raw ambition. It’s best to acquire your charisma (which the author annoyingly calls ‘rock star’ status) only after winning power with modesty, Richards suggests, as Thatcher and Blair did. Good point.

Michael Portillo suffered the early taint of ambition, as did David Miliband (who shares a chapter here with his fratricidal brother, Ed). Both fatally hesitated when called to plunge the dagger into, respectively, John Major (in 1995) and Gordon Brown (in 2009). Installing telephone lines and writing challenging Guardian articles are mere chocolate blades. Gallant, doomed Neil Kinnock (‘Christ! What a way to spend my forties,’ he would lament about his time as Labour leader) was beset by self-doubt. So was Portillo, whose core identity seemed as fluid as his sexuality. Was he an acolyte of Harold Wilson (his teenage hero), a ‘Who dares wins’ Thatcherite SAS man or a liberal Tory wet? Thatcher’s anointed heir (‘We expect great things of you, you will not disappoint us,’ she said at his fortieth birthday party) ended up making telly travelogues in lurid jackets. Oh, dear. At least Corbyn has retained the dignity of tending to his allotment in his retirement years.

Richards likens Corbyn’s accidental elevation after the shock defeat of Ed Miliband in 2015 (not a shock to me) to a weekend tennis player being thrust into the Wimbledon championships yet showing no inclination to learn new shots. A lifetime preaching solely to the converted is no training for winning new adherents, Richards says. I would say that the contest for No 10 is more of a penalty shootout than a tennis match. Life is lonely and brutal for those who miss.

Richards writes like the Westminster insider he is. So he presumes to tell Roy Jenkins, no less, that the great man was ‘not an entirely accurate reader of his own political mind’. Although now a practised columnist, Richards started out as a broadcaster. He has interviewed most of the subjects of this book (as he did those of his previous book, on actual PMs) and brings to it the wholesome broadcaster’s perspective. This means he underplays the influence of the Tory-supporting, oligarch-owned tabloids in the murkier corners of politics. There are lots of shrewd insights here, but the book would have been better with slightly fewer of the author’s emollient ruminations and more colourful material about his subjects. Less earnestness, more jokes please! Richards is, after all, also a stand-up comic.

Hovering over these speculations is surely Tacitus’s scathing verdict on Galba, the first of the three emperors to succeed Nero, all of whom were promptly bumped off in the chaos of AD 68–9: ‘Capax imperii nisi imperasset.’ In other words, Galba looked like emperor material until he became emperor. Some biographers of nearly PMs write love letters to their fallen heroes. Richards is more concerned with why his chosen picks lost, but he does not answer the Galba question: would they have been any good? He senses Kinnock and Hezza might each have steered a significantly different course for Britain, away from Euroscepticism. It is also possible, as in the cases of the Tory moderniser Butler and the social reformer Jenkins, to have substantial careers without ever reaching the peak. To understand quite how Westminster’s talent pool shrank from Butler to Boris, we must await Richards’s next book.