Image copyright Reuters Image caption Yulia Skripal and her father, Sergei, were poisoned in March

It is becoming clear, without any official policing sources confirming it, that significant progress has been made in the Salisbury spy poisoning investigation.

Detectives from the South East Counter-Terrorism Unit appear to have got to the stage where they have isolated two people from CCTV images as their prime suspects in the suspected nerve agent poisoning of Sergei and Yulia Skripal in March.

The attack led, three months later, to the subsequent death of Dawn Sturgess, and the accidental contamination of her partner Charlie Rowley.

At this stage no-one has confirmed officially that the two suspects have been fully identified by their real names, but a discussion has begun between counter-terrorism officers, the Crown Prosecution Service, the Foreign Office, the Home Office and the intelligence agencies as to what is the best way forward.

The key question in those discussions is whether the UK should make a formal request to Russia for the two people suspected of carrying out the Novichok attack to be sent to the UK for trial.

We know from the case of Alexander Litvinenko, killed in London in 2006, that such a request would be unsuccessful, because Russia would simply refuse.

In the Litvinenko case Russia said it could not, under its constitution, send Russian citizens for trial in the UK.

The two Litvinenko suspects - Andrei Lugovoi and Dmitry Kovtun - are still both living as free men in Russia, although neither can leave the country, as they know they risk being arrested on behalf of the UK.

Andrei Lugovoi has made a successful career as a member of the Duma, the Russian parliament.

He has made a virtue of enjoying visiting all the wildest parts of his huge country instead of travelling overseas.

On one of the times I met him he was fishing and horse-riding in Kamchatka, a remote volcanic peninsula in the far east of Russia.

Image caption Daniel Sandford (l) met Andrei Lugovoi (r) on a fishing trip in eastern Russia in 2011

So, making an extradition request will produce a firm "nyet" from Moscow, but would at least allow the UK to show that it has identified two suspects.

It would not help relations between the UK and Russia although these are at an all-time low anyway since the break-up of the Soviet Union.

However, not making a request would leave Russia guessing and mean that the two suspects would not know if they had been fully identified and would leave them permanently worried about leaving Russia.

This may seem like quite an attractive option, but would allow Russia to continue to say that the initial British government accusations were baseless, and not supported by any evidence.

A third option is to do what ultimately happened in the Litvinenko case, and hold a public inquiry in which as much as possible of the evidence is laid out for everyone to see.

Of course, this third option does not rule out making an extradition request initially, and then proceeding to a public inquiry when the request is refused.

Whitehall officials continue to say that this is a police investigation, but ultimately some political decisions will have to be made about how to proceed.