Despite barely managing more than a croaky "thank you" during the whole of the series finale, King George VI plays a pivotal role in SS-GB.

But who is the pallid actor wrapped up in a woolly balaclava to play His Sickly Royal Highness, the monarch at the centre of a plot to free him from hospital and fly him off to America? That would be Jonathan Cullen.

If he looks familiar, that might be because Cullen has an excellent line in TV and movie bit parts. He was the headmaster in Outnumbered, a prison doctor in Suffragette, Dr Plant in Inspector George Gently and Jeremy Ventham in New Tricks. Or if you're a theatre fan you might recognise him from roles in Equus, Dr Faustus, and The Merchant of Venice. 

Was King George VI really ill during World War II?

Obviously SS-GB is a counterfactual version of World War II dreamed up by novelist Len Deighton, so the monarch's fate has taken a dramatically different direction unrestrained by real-life events.

But it's a bit of a surprise to see King George so unwell (and it seems to be a surprise for Harry and Archer as well, who exchange worried looks as they bundle him into the ambulance to begin his journey).

In this alternative reality the monarch was kept in the Tower of London while his wife and daughters escaped to New Zealand: he has now become extremely sickly. Though this is barely explained in the TV series, there are hints in the novel that he may have sustained brain damage during the invasion: 

"It must have been that bomb that hit the palace just before the end," said Harry in a whisper. "There were rumours that the King had been badly injured, do you remember?"

"You think he's been like this all that time?"

"I've seen plenty such cases," said Harry. "It's the concussion - the blast effect can kill without leaving a mark on the corpse. Or it can just numb the mind and shake a man's brain loose." 

Certainly, in real life King George (affectionately nicknamed Georgie by Sylvia) was not a long-lived man. But he made it to 1949 before serious health problems set in. That was the year when he suffered an arterial blockage in his leg, but by 1951 his health had deteriorated further: his left lung was removed when a malignant tumour was found. Finally he died in his sleep in 1952 at the age of only 56.

However, during the World War itself, George seemed perfectly healthy. He and his wife Queen Elizabeth toured hospitals and appeared in Britain's bombed-out cities, and in 1943 he visited British troops in North Africa to boost morale. He also visited troops in Malta and – 10 days after the D-Day invasion in 1944 – went over to Normandy to visit the troops. 

So why is the King so sickly in SS-GB? It seems injuries sustained during his capture and the occupation have hastened his deterioration. But the way Cullen plays him, he cuts a pitiful figure: the pawn in other people's political games, reduced to a hunched man in a beige balaclava.