There could hardly have been a more brazen statement of intent. And when Sergei Skripal and his daughter Yulia were found close to death on a park bench in Salisbury on March 4, 2018, there was one blindingly obvious conclusion to be drawn – that Russian President Vladimir Putin had just tried to assassinate a well-known enemy of the Russian state on British soil.

Theresa May, then Prime Minister, was in no mood for uncertainty, declaring to the House of Commons that the Russian Federation was directly responsible for ‘an attempted murder here in our country… an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk’.

The evidence pointing towards Moscow seemed clear. For a start, Skripal looked to be an obvious target.

Both Sergei Skripal and Yulia Skripal survived one of the most toxic agents known to man. Former MP Norman Baker writes: 'I am in no doubt that the Russians are engaged in highly aggressive practices against the interests of this country'

A former member of Russia’s GRU military intelligence agency, he had spied on behalf of Britain for several years before he was caught and sent to a Siberian jail. He arrived in Britain as part of a prisoner swap.

Then there was the nerve agent said to have been used in the attack: Novichok is a rare and especially deadly poison developed in a Russian laboratory, we were told.

Later, we would learn that two Russian agents had been visiting the cathedral city at the time of the attack. Convincing as it might sound, however, this widely accepted account of the events in Salisbury does not add up.

Had it been submitted to a publisher of fiction, it would have been rejected as having more holes than a Swiss cheese.

Too many of us assume that when categorical statements about fact and blame come from the mouth of a Prime Minister, no further questions are necessary or even appropriate.

Ruslan Boshirov and Alexander Petrov are pictured in Salisbury in March 2018. Indeed, they seemed almost at pains to be caught on CCTV cameras as they went window-shopping

That applies to MPs and even to some journalists. Complacency, after all, had been the near-universal response when Tony Blair presented hyped-up dossiers to the House of Commons to support his now-notorious claim that Saddam Hussein possessed weapons of mass destruction that could be activated against us within 45 minutes. There were no such weapons.

I am in no doubt that the Russians are engaged in highly aggressive practices against the interests of this country.

There have been cyber attacks, for example, and attempts to manipulate public opinion through disinformation on social media.

It is true, also, that Russia has form for killing critics and traitors, even when the targets believe themselves to be safe on foreign soil.

Only last week, Germany expelled two Russian diplomats after a former Chechen rebel commander was shot dead in Berlin amid claims the killing had been ordered from Moscow.

The Russians have certainly used Britain as a location to murder opponents of the Putin regime.

In 2006, Moscow passed new laws giving its agents licence to go abroad and kill enemies of the state.

Most people will know about Alexander Litvinenko, who died in London in 2006 after a Russian agent added radioactive polonium 210 to his tea.

His was one of 14 deaths in this country which, according to US intelligence agencies and From Russia With Blood, a new book by investigative journalist Heidi Blake – serialised in last week’s Mail on Sunday – bear the hallmarks of a Kremlin assassination.

Typical of the cases was that of Scot Young, a fixer who meddled in Russian finance. He had gone to the police in 2014 to say he was being targeted by Russian hitmen.

The Novichok was said to have been in a gel form, but would Skripal not have noticed this when he pulled the door to? He was, after all, trained in spycraft back in the 1970s. In any case, how many people does it take to close a door? Police are pictured above outside his house

When his body was found soon after impaled on railings four floors below an open window in Marylebone, they declared it suicide and did not even bother to check for fingerprints. Nor did they appear to notice the scratch marks in the dust on the ledge outside, from fingernails desperate to hang on. Case closed.

Then there was MI6 agent Gareth Williams, who had been working on highly sensitive Russian leads related to money-laundering. He was found dead in his Pimlico flat in 2010, padlocked into a sports bag with the key under his body.

The bag had been placed in the bath but there were no fingerprints or DNA from Mr Williams on the bath-tub, the zip on the bag, or the padlock used to secure it.

An expert in confined spaces called Peter Faulding said he tried 300 times to lock himself into an identical bag but failed.

And the heating in Mr Williams’ flat was turned on full blast even though it had been middle of summer – an act which would have hastened the body’s decomposition in the ten days it had lain there. The police concluded his death was ‘probably an accident’.

Why would the British play these matters down? Was it a wish not to endanger the Russian billions flowing into the City?

Perhaps a report on allegations of Russian interference in British politics by the Intelligence and Security Committee will tell us when it eventually appears.

Although cleared for publication by the security services, it has been withheld by the Prime Minister until after the Election. None of this, however, means that what we have been told about what took place in Salisbury is accurate.

The investigative site Bellingcat later unmasked them as Ruslan Boshirov, a highly decorated military officer, and intelligence officer Alexander Petrov. They later gave an interview to Russian television in which they suggested they had been visiting Salisbury to see the cathedral

The first of those questions is this: why would Russia want to assassinate Skripal eight years after he first arrived in Britain? He was retired and posing no obvious threat or provocation to Moscow.

It is said that the Russians never forget a defector, but Skripal was part of a swap, and in the world of international espionage, there is a long-standing convention that you leave alone those you have agreed to exchange.

There is another curious aspect to Skripal’s case: according to ex-Kremlin official Valery Morozov, Skripal had been making regular visits to the Russian Embassy in London, probably because he wanted permission to return home to see his relatives.

If Putin wanted to kill him, why not get him back to Russia first? As for his daughter Yulia, she had only arrived in Britain 24 hours earlier and had been living in Moscow for years.

If they wanted to ‘nail’ her too, why did they wait until she landed in this country?

Then we are told that the poison chosen to dispatch the Skripals was Novichok, described by Theresa May as ‘a military-grade nerve agent of a type developed by the Soviet Union’.

This is true, but stocks of it are held not only in Russia. Germany managed to secure a sample back in the 1990s from a Russian scientist, and duly made this available to a small group of allies, including Britain, to enable testing of protective equipment.

This country’s supplies are kept at the Government’s Porton Down laboratory, a mere seven miles from Salisbury.

It is a site which, for all its undoubted sensitivity, does not have an unblemished security record. In July, 30 years of secret Porton Down documents were found dumped in a London bin.

The Russians have been getting away with murder – literally. Yet as far as events in Salisbury go, it is obvious the explanation we have been given by those in authority does not stack up

The availability of Novichok is not the only puzzle. When the Russian government asked for a sample of the nerve agent said to have been used to attack the Skripals, none was provided by the British. If the agent in question had been administered by the Russians, why not agree to the request?

Novichok is hugely potent, some five to eight times more toxic than the VX military nerve agent.

We are told that the batch used on the Skripals was of a high purity, up to 97 per cent, a level that can only really be achieved by a state actor.

We also know that Novichok was designed to take effect rapidly, within two minutes.

Yet according to the official story, the Skripals were poisoned by Novichok that had been smeared on the door handle of their house, when they left at about 9.15 in the morning – more than six hours before they fell ill.

In the meantime, they had driven around town and visited both a pub and a restaurant, all seemingly without any ill-effects. This is perplexing.

The Novichok was said to have been in a gel form, but would Skripal not have noticed this when he pulled the door to?

He was, after all, trained in spycraft back in the 1970s. In any case, how many people does it take to close a door? How did both become ill, and indeed both simultaneously on that park bench, several hours later?

I asked the Metropolitan Police if CCTV had been installed at the Skripals’ house, surely a basic security requirement for someone in his situation. They were unable to give me an answer.

No footage has been issued showing anyone near the door handle. We do know that subsequently, the roof was replaced on the Skripals’ house, supposedly due to Novichok contamination. From the door handle?

The emergency services were called to the park bench at 4.15 that Sunday afternoon, and arrived within minutes. But the Skripals were only taken to the local hospital about an hour later.

It seems that everyone waited while the Skripals were attended to by a doctor who happened to be first on the scene.

We were not told at the time who this was, but almost a year later it emerged that she was the Chief Nursing Officer for the British Army, Colonel Alison McCourt. What an amazing stroke of luck!

Only last week, Germany expelled two Russian diplomats after a former Chechen rebel commander was shot dead in Berlin amid claims the killing had been ordered from Moscow. Forensic experts are pictured above at that crime scene

In her statement to the Commons, Theresa May stated that ‘more than 130 people could have been potentially exposed to the nerve agent’. She added that more than 50 had been assessed in hospital.

Given the nature of Novichok, it is indeed fortunate that nobody died from exposure to this highly toxic chemical – not the Skripals, who I am assured are now both well, nor Col McCourt, despite having been in close proximity to them without wearing protective clothing.

Interestingly, Dr Stephen Davies, the local consultant in emergency medicine, felt obliged to write to The Times to state that ‘no patients have experienced symptoms of nerve-agent poisoning in Salisbury and there have only ever been three patients with significant poisoning’.

This actually implies that, whatever was used to poison the Skripals – and Detective Sergeant Nick Bailey, a police officer at the scene – it had not been Novichok.

Tragically, one person did die from a separate incident several weeks later. Dawn Sturgess, a local resident, was given a perfume bottle by her partner Charlie Rowley.

This had been reclaimed from a charity bin, situated coincidentally near to the park bench. We are invited to believe this was thrown away by the Skripals’ assailants, as it was said to contain an identical chemical.

Yet it contained a spray rather than a gel. Who would go to the considerable trouble of developing two separate delivery mechanisms?

It also transpired that the bottle was wrapped in cellophane, as if new. Is it really true, moreover, that the bin had not been emptied for nearly four months?

Undoubtedly, there were at least two Russians in the city that day. Indeed, they seemed almost at pains to be caught on CCTV cameras as they went window-shopping.

The investigative site Bellingcat later unmasked them as Ruslan Boshirov, a highly decorated military officer, and intelligence officer Alexander Petrov.

They later gave an interview to Russian television in which they suggested they had been visiting Salisbury to see the cathedral.

If they were sent to dispatch the Skripals, they were grotesquely incompetent – unlike those responsible for the earlier suspicious deaths.

The Russians have been getting away with murder – literally. Yet as far as events in Salisbury go, it is obvious the explanation we have been given by those in authority does not stack up.

We live in a mature democracy, and we are entitled to the truth. Let us now have it.