What would it take for you to betray your country?

Fortunately, it’s a question few of us will ever be asked.

But 80-odd years ago, five undergraduates at Cambridge University faced that question - and found their own answers for their perfidy.

Alan Bennett’s two stories in one play, Single Spies, which opened last night at the Marlowe Theatre, deals not so much with the reasons for their treachery but the consequences.

Two of the “Cambridge Of Spies”, Guy Burgess and former Surveyor of the Queen’s Pictures, Anthony Blunt are the protagonists in these back-to-back one-act plays, first staged at the National a quarter of a century ago.

Blunt and Burgess were part of the sinister coterie of well-educated, well-to-do dons and students at Cambridge in the 1930s, which also included Kim Philby, Donald MacLean and John Cairncross.

The first of the plays ‘An Englishman Abroad’ is set in Moscow in 1958 and based on a real encounter between the drunken spy – now exiled in Russia – and Australian actress Coral Browne.

Nicholas Farrell is a superbly chaotic Blunt, disheveled, constantly drunk who has staggered into Michael Redgrave’s dressing room following a performance of Hamlet by the Shakespeare Memorial Theatre and promptly thrown up in his sink.

Later, a bemused Browne recognises him as one of the Russian spies who had fled the UK in 1951 and accepts a written invitation – slid under her door - to visit him at his sparse flat, the note ending with "and bring a tape measure".

It’s here during an even sparser lunch of tomatoes – which Burgess eats both while downing copious amounts of alcohol – that Farrell captures the essence of the pitiful, miserable spy who misses London life.

When she arrives she asks: “What’s that smell?” The spy, dressed in a suit which like him had seen better days, retorts: “Probably me!”

He begins asking about a stream of people Corale – played by Belinda Lang – has never heard of before playing his one and only record by Jack Buchanan.

The louche Burgess asks diffidently: “Do you know him?” The actress replies: “I suppose so... we nearly got married!”

The play is Bennett at his best, full of pithy and poignant asides delivered fabulously by Ms Lang as she tackles why the former BBC and Foreign Office worker had turned his back on Britain for Stalin’s Russia.

Like all spies, Burgess never gives a straight answer but it’s clear he is a prisoner in his own mind, in the life he had chosen and in his sordid flat – and Farrell leaves you in no doubt that this spy is conning no one anymore.

Then we see why the amoral Burgess, who had stolen soap and cigarettes from Ms Browne’s dressing room with the same aplomb he stole Britain’s secrets, has asked for a tape measure – because he wants the actress to order him a new Savile Row suit, silk PJs and a new Eton tie.

In A Question of Attribution, Ms Lang pops up as HMQ in an imagined encounter with Anthony Blunt at the Palace in the 1960s.

The two are then engaged in a discussion on art, fakes and forgeries – at a time when Blunt’s treachery was know the Secret Service and in real-life by the Queen.

Because the 2Point4Children star doesn’t resemble Her Majesty and makes no attempt to caricature her, we are able to concentrate on Bennett’s brilliant script.

This is an all-knowing and clever interrogator Queen who persists with her questions in the same clever way Nicholas Farrell does playing the secret service officer Chubb, who is questioning the former Cambridge don on his duplicity.

The snobbish art historian - played wonderfully by David Robb - is then left floundering by the incisive apparently innocent inquiries on fakes.

He argues: “Because something is not what it is said to be, Ma'am, does not mean it is a fake. It may just have been wrongly attributed.”

Like Ms Browne, the Queen can also spot a fake.

A protégée asks what Blunt – who would be exposed as a spy in the 1970s and stripped of his knighthood – and the Queen had been discussing.

He answers: "I was talking about art. I'm not sure that she was."

Three great characters, two great plays, and one superb playwright makes it a joy to watch.