On the evening of 28 June 2010 New Zealander Henry Frith, a business consultant, was approached by a stranger outside his apartment in Madrid, asking if he had a few minutes to chat. The man, who was British, stressed that he thought it was very important for Frith that he complied: “You see, I have your life in my hands.”

The visitor went on to explain that there would be a “big problem” for Frith if he did not talk now.

The man calmly continued that he worked for British intelligence and that he knew full well that Frith worked for Russian intelligence. He apologised for surprising him out of the blue, but explained that it was the only way to talk securely. The offer was simple and one that is age-old in the world of espionage: to be a double agent. Working with the UK would be an “opportunity to make your life a lot better”, the British officer assured.

Frith repeatedly insisted that it had been a case of mistaken identity. He had nothing to do with spies and he was not Russian. He was born in Ecuador: his mother was Ecuadorian and his father from New Zealand. He specialised in socio-economic aspects of investment.

The meeting ended with the British man saying he would keep in touch.

Next morning Frith flew out of Madrid. He next surfaced in Moscow where Sergei Cherepanov, which was his real identity, was welcomed back by his colleagues in the SVR, the Russian foreign security service, and reunited with his wife Olga and son, Andrei.

Two Russian diplomats based in Madrid, Anton Olegovich Simbirsky and Aleksandr Nikolayevich Samoshkin, were subsequently accused of “running” Cherepanov and expelled from Spain.

The details of how Cherepanov was approached by Western intelligence, and the conversation which took place, came later from Spanish officials. The attempted recruitment had come a day after the FBI had arrested 10 Russian agents in Boston, New York and Washington. They were “sleepers”, so called “illegals”, who had been in the US since the 1990s pretending to be Americans.

In June 2011, a former SVR officer, Colonel Alexandr Poteyev, who had like Sergei Skripal had served with the military in Afghanistan during the Russian invasion of the country, was sentenced to 25 years in absentia for betraying the Anna Chapman network and Cherepanov.

There was huge publicity over the case, a lot of it focusing on the glamorous Anna Chapman, born Anna Vasilyevna Kushchyenk, a Russian citizen who had obtained British citizenship by marriage. She was later freed in a spy-swap under which a Russian military intelligence colonel who had become an agent for MI6 – Skripal – was freed and settled in England with his wife.

He disappeared from view until, as we know, the extraordinary murder attempt on him and his daughter Yulia with a nerve agent in his adopted hometown of Salisbury. Last weekend Chapman, who is now a Kremlin poster girl, model and TV presenter, condemned Skripal on social media as a traitor.

The Spanish connection runs through the Skripal story. British and Spanish security agencies have been in liaison since the poisoning in Salisbury, according to a number of sources. Officials from both countries, however, refused to comment on the matter.

Skripal was posted to Madrid by Russian military intelligence, GRU, in 1993. Cherepanov was already there as Frith (using the name of a New Zealand child who had died young, a classic ploy to create a “legend” in spycraft) for three years. Skripal, according to reports, had met Cherepanov in Spain. Yet his name could not have been passed on to MI6 among those that Skripal supplied to his British handlers. If it had been then London would have informed Madrid of the spy. Alternatively, the British were told about Cherepanov, but did not pass on the information to their Spanish allies.

There is a possibility that the two branches of Russian intelligence, SVR and GRU, were so much out of touch with each other that they were unaware of each other’s agents. But this means accepting not only that Skripal did not know Cherepanov, but was also unaware of the activities of the handlers Simbirksy and Samoshkin despite being based in the same embassy.

Intelligence analysts are sceptical about this scenario, especially at a “station” like Madrid where there would not be that many undercover Kremlin operatives, unlike Washington or London or Paris. There were certainly marks of official embarrassment over what had happened; it took a long time for the details of the affair to emerge in Spain and Spanish security agencies to acknowledge that the spy had been at work.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

A Russian documentary made with the help of the country’s intelligence services charted Skripal’s time in Spain. His “turning” by Western intelligence, it described, began with an agent with the Spanish intelligence service, calling himself Luis, befriending him.

It was already known that Skripal was looking for opportunities to make money. Luis suggested that the two men start a private business, exporting Spanish wine to Russia. After a while more money was needed and Luis introduced the Russian colonel to an MI6 officer who would become his handler. From then on the Russian would supply information to the British in return for money. Cherepanov, meanwhile, continued with his double life in Madrid, running his company Frimor Consultores with a Spanish partner and carrying on spying for Moscow guided by Skripal’s diplomatic colleagues Simbirksy and Samoshkin.

At the end of the Nineties Skripal returned to Moscow to work at GRU headquarters. He continued to supply information to MI6 in return for money which became a fairly consistent demand. At one point he sent an emergency message to London. There was consternation that the Russians may be closing in on him and there would be a need for extraction. Instead it transpired that Skripal was urgently demanding $10,000 (£7,200): the money was sent to him.

Skripal would often fly over from Moscow to collect his fees in Spain. However, he suffered from diabetes, and sometimes it would be his wife Lyudmila who would collect the money instead. Lyudmila had always maintained that she thought the money was from his private business activities. But did the Kremlin disbelieve her? Could that be the reason that she, too, may have been targeted, if indeed it transpires that her death was not due to natural causes, endometrial cancer?

The end to Skripal’s life as a British spy came, as it had begun, in Spain when a Russian agent inside the Spanish intelligence service, Roberto Florez Garcia, discovered the military intelligence colonel’s double-life and told his bosses in the Kremlin. He was convicted of treason and sentenced to 12 years in prison by a Spanish court.

Skripal was arrested outside his home in Moscow in December 2004 in front of state TV cameras. He was convicted two years later of treason after being charged with betraying dozens of his comrades to MI6 and was sentenced to 13 years in prison. Others accused of being traitors in Russia had faced far worse fates: among them Oleg Penkovsky, a colonel in the GRU like Skripal, who was executed for spying for the British and the Americans, and Dmitri Polyakov, a major-general in the GRU, who also faced the firing squad. Moscow announced that one reason for the relatively lenient sentence was that Skripal had cooperated with the authorities.

Aleksandr Poteyev, who had betrayed the Anna Chapman network, had fled to the US before his trial. Vladimir Putin declared that traitors would be hunted down: “No matter what 30 pieces of silver these people received there, they will have a stake in their throat, I assure you. To hide their whole lives, not to have the opportunity to talk to their loved ones. You know a person who chooses such a fate will regret it a thousand times.”