The man who is arguably the most powerful force in the land at the moment was a stranger to many in Westminster until last summer.

Geoffrey Cox, a more familiar face at the Bar until his appointment as attorney general in July, now holds the fate of Theresa May's premiership in his hands.

The prime minister has placed Cox in charge of negotiating changes to the Northern Ireland backstop in the EU withdrawal agreement.

If the attorney general succeeds, he can expect the prime minister to be eternally grateful. He would have pulled off the seemingly impossible after her Brexit deal was comprehensively rejected by MPs in January.

Conservative MPs are bracing themselves for a flamboyant Gilbert and Sullivan performance by Cox at Westminster if he lands a deal with the EU.

With his distinctive rich baritone voice, Cox is being lampooned by some Brexiteers as a modern Pooh-Bah, the character in the comic opera Mikado who amasses numerous titles and is known as the Lord-High-Everything-Else.

The Brexiteers say that, like Pooh-Bah, Cox has amassed jobs that are bound to run into conflict with each other. He is now the government's chief negotiator with the EU on legal changes to the Northern Ireland backstop while serving as the government's chief law officer.

The multiplicity of roles means that Cox will conduct the negotiations and then pass legal judgment on them.

One Brexiteer tells me: "People think the attorney general is this great independent force. But the government is his client and he is negotiating the deal on its behalf. But he will then mark his own homework. He will become his own client."

The DUP are quite attracted by the idea of using the transition period, in which the whole of the UK would effectively remain as a non-voting member of the EU, as a way of sidestepping the backstop.

The DUP demand would involve making changes to an early version of the withdrawal agreement which was subsequently tweaked by EU leaders at a summit on 25 November. The final version reiterated the December 2020 end date for the transition, but added that it could be extended "for up to one or two years".

The EU is insistent that it will not amend the text of the withdrawal agreement. But it has indicated that it would be willing to provide legal assurances as long as they do not contradict the agreement.

That lies at the heart of the complex set of negotiations being conducted by an attorney general, who has embarked on what he regards as a deadly serious mission.