As the final episode of SS-GB unfolded on BBC1on Sunday night, fans of Len Deighton's alternative history novel were in for some surprises.  

Cramming the murder mystery's complex plot into five hour-long episodes must have been a hard task for former Bond writers Neal Purvis and Robert Wade. To adapt if for TV they made some major changes, not least (spoiler alert!) keeping Barbara alive, removing Harry's role as an informant, and giving Sylvia a friendship with the King.

Take a look at the spoiler-filled breakdown below to find out six key ways the adaptation was different from the original:

Sylvia still dies – but at a completely different moment  

Okay, so Sylvia Manning (Maeve Dermody) was always going to die: she was a marked woman from the start, a disposable character who nonetheless displayed plenty of guts (you have to admire her for setting fire to a box full of yellow stars at Scotland Yard and legging it out the door). Accordingly she dies in both the novel and the TV show – but at completely different times.

Novel-Sylvia and TV-Sylvia's paths diverge when she and Harry try to break out of detention. Instead of dying as she was originally intended to (from the book: "Harry was in a shooting incident this afternoon... the girl with him – Sylvia Manning, who used to be your clerk – is dead"), the fearless Resistance fighter survives her getaway attempt with only an arm wound and turns up at Archer's flat to camp out. She then comes to the rescue of Archer, Harry and the King when their getaway ambulance has mechanical trouble, decides to tag along for the ride ("there's nothing for me here"), bonds with King Georgie, fights off a family of collaborationists and finally gets all the way to Bringle Sands. But in the SS-GB version of Final Destination, death catches up with her and Sylvia is fatally shot alongside the King. 

Barbara escapes the grim reaper!

This might be 'Final Destination: SS-GB', but Barbara Barga (Kate Bosworth) actually manages to cheat death altogether in the BBC series. With Sylvia already dead, Deighton's novel sees Archer turn to Barbara to save the getaway operation when the ambulance breaks down – but he's perplexed when she tells him on the phone not to come over. Ignoring her veiled warnings, he and the gang head over anyway, let themselves in and call Mayhew for help. But then Harry discovers her in the bedroom, dead and "knocked about a bit". 

In the TV series, the Germans actually visit and attack Barbara before the operation to free the King gets under way, so when Archer bursts into the room the day before, it's been ransacked and the phone is smeared with blood – but there's no gruesome body waiting for him. Instead, she's taken into custody, terrorised, interrogated by Kellerman, and finally rescued by the American Embassy's legal attaché. Barbara is due to be deported home, but in a twist at the very end of the series she locks her fellow countrymen in a room and heads out onto the mean streets of London.

Archer doesn't meet Barbara's husband

In the TV series, Barbara is coy about whether she's already married or not – but after a romance develops between her and Archer, the walls come down and she reveals a plan to marry him and take him back to America with his little son Dougie in tow. 

But in the novel, we actually meet her American husband Danny Barga, who parachutes in and becomes involved in the escape attempt. Recognising Archer, he says: "You're the goddamned Scotland Yard dick I've heard so much about," before adding: "Barb said I would like you, and dammit, I do." Archer's not sure if the feeling is mutual. Either way, Danny is dead before the end of the story. 

Huth makes a deal with Mayhew – but Archer has no idea

The TV series saves all the big reveals for the end, delivering them with a big whomp.

When Archer and co arrive at Bringle Sands, they find Huth and his men waiting for them. Sylvia and the King are shot to death, Harry is injured and Archer is knocked unconscious.

But Huth and Archer quickly realise they have both been double crossed: the plot to free the King was a diversionary tactic engineered by Mayhew, keeping Huth distracted and making the King a martyr while the real battle was taking place at the nuclear research centre on the coast. In a heart-to-heart at an abandoned country mansion, Huth reveals how Mayhew made a deal and span him a story: he'd been told that the plot was actually to fly top nuclear scientists out of the country, and that's who he thought he was ambushing. 

So what happens in the novel? Huth's deal with Mayhew actually takes place when the German Standartenführer turns up, suspicious about Danny Barga's parachute, and tries to arrest them all. Archer threatens to shoot him, but Mayhew takes him off for a quick chat – and that's when the double-crossing happens. It's much more obvious to Archer – and the readers – that Huth knows something is afoot. Archer also knows that Huth has had contact with Mayhew. 

Archer is not arrested at the end

In the SS-GB TV series, after the pivotal moment where the King is shot dead, Huth is the one who takes his protege Archer off for a chat. This is when Archer finally twigs that Mayhew set everything up from the beginning, murdering Dr Spode as part of a masterplan to draw America into the war. The two of them know that Kellerman is on his way and Huth is resigned to his defeat, even offering Archer the chance to arrest him and be a "hero" to his German bosses.

Our protagonist of course declines and makes himself scarce (with the help of a priest hole), leaving Huth to be arrested by Kellerman and executed by firing squad. 

Strikingly, this is a major diversion from the plot of the novel. Huth is nowhere to be seen in the aftermath of the attack, and Archer ends up in jail at Bringle Sands. Harry tries to get him out but fails. In the morning, Archer receives a visit from Kellerman, who outlines how Huth (his bitter enemy) was to blame for the whole plot, adding: "Ah, you did not see what was going on, my dear Superintendent. You are a fine and loyal officer, and no blame could ever attach to you." Kellerman doesn't want any of the blame for missing his Superintendent's scheming, so Archer's off the hook. 

At the end of the drama adaptation it's pretty clear that Archer is striking off by himself, leaving Scotland Yard behind as he flees – though his precise plan is never revealed. But the novel is more coy. The only clue we get is this: "Douglas picked up Mayhew's duffel coat from where he'd left it on a chair. He put it on and closed the wooden toggles. It would be cold outside, and he'd be grateful for this ill-fitting coat."

Harry Woods is not an informant 

The novel turns the status of Sergeant Harry Woods on its head. At the beginning, Archer realises Harry is a Resistance man embedded deep within Scotland Yard, and isn't sure if he can trust him. But finally Huth tells Archer that, in the end, Harry was Kellerman's man: "Harry Woods agreed to be Kellerman's informant, telling him every move you made, every meeting that took place, reporting every word to which he could get access." 

Why? Out of love for Archer and little Dougie. "Don't you realise that Harry Woods looks upon you as the son he never had?" asks Huth. It's his parting shot as he heads out to face the firing squad. And then it's all over.