Man down! Steven Knight’s crackerjack dramatisation of the formation and second world war exploits of the Special Air Service returns with a snag to fix. In season one, this new elite British army regiment, and the show commemorating it, were led by Connor Swindells as David Stirling, an officer with an intoxicating blend of debonair dash and suppressed rage. But SAS: Rogue Heroes is based on fact, albeit in a scatter-gun, tie askew, stolen-your-wife sort of way, and the fact is that in January 1943, Stirling became a prisoner of war in Italy. Season two begins in the spring and summer of that year, so Stirling is duly confined and Swindells, the star, is hardly in it.

Taking command now is Jack O’Connell as Paddy Mayne, previously the screaming id to Stirling’s smart ego. Can he hack it on his own? The success of the operation depends on him … Paddy Mayne is still a borderline-certifiable risk-taker and dead-eyed slayer of the faceless enemy, but his new position as a leader requires a new level of reckless bravery from the actor playing him. O’Connell is equal to the task.

Having helped to gain control of north Africa in the first season by taking on ridiculously dangerous missions behind enemy lines, the fledgling SAS are given what London-based strategist Dudley Wrangel Clarke (Dominic West) drolly describes as “a new and even more effective way of committing suicide”. The men, temporarily rechristened the Special Raiding Squadron for tricksy army-admin reasons, are to lead the southern extremity of the invasion of continental Europe, starting with a landing on Sicily that the top brass anticipate will have a fatality rate of 50%.

Before all that, a postman bicycles calmly through a country lane in County Down. The letter he delivers is from Paddy Mayne, writing home to tell his mother that his commanding officers have not allowed him compassionate leave to attend his father’s funeral. We see Mayne’s reaction to this in an early scene where he smashes up a posh Cairo restaurant and the five military policemen who try to stop him breaking any more chairs.

The second world war, with its unimpeachable casus belli, is safe ground for a drama about military heroism – certainly you wouldn’t be able to dramatise the postwar adventures of the SAS without running into thick ethical murk and a lot of icky soldiering fantasies. But one can enjoy SAS: Rogue Heroes without fear of turning into Alan Partridge salivating over an Andy McNab hardback. Yet the series is alive to the class divisions within its cast: even with a cause as unifying as stopping the Nazis, there is still a strong element of posh men telling working-class lads where and when to die.

As played by O’Connell, Paddy Mayne is the ideal hero in this scenario. His snarling rejection of authority is absolute and, in the second series, he has a fascinating new ally/antagonist in the form of Bill Stirling, David’s older brother. The cause of some real-life debate about which Stirling should be credited with the establishment of the SAS, Bill introduces himself to Paddy as the more diligent, less impulsive sibling, but his self-image is about to take a battering. Paddy greeting Bill by staring him down and telling him he sees no reason to pay him any respect is one of several moments where O’Connell makes us want to punch the air.

As Stirling tries to bring Mayne around by pointing out their shared goals, Gwilym Lee is excellent at bringing out the supposedly more refined and capable man’s mixed emotions. Does Bill Stirling feel fear, admiration, envy or patrician disdain as he negotiates with a man who personifies the reckless courage his side will need to win? Lee gives us all of them at once. For his part, O’Connell is his own whirl of conflicting strengths and weaknesses as he briefs the SAS rabble before the trip to Europe, struggling to accept his role as their superior but eventually winning them over – and becoming the man he needs to be before our eyes – by acquiring a sudden, steely stillness and quoting William Blake: “Prepare to meet our fathers in the sky … ”

With its sober view of sacrifice and its clever use of extreme adversity to bring out different facets of the male psyche, Rogue Heroes earns its stripes, but be reassured that with all that groundwork in place it is, primarily, a right old romp. The wildly incongruous but perfectly chosen rock soundtrack, the cool freeze-frames, the fist fights, the banter and of course the battle scenes are all unashamedly exciting – and the Sicily landing, with Mayne’s men dodging Italian bullets under powdery grey moonlight, is as cool as hell. It’s a thrill to watch O’Connell and his boys charge on.

SAS: Rogue Heroes aired on BBC One and is on iPlayer now.

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