On a freezing, miserable, dank evening back in January, Martin Mc Gartland emerged out of the shadows to meet me in a corner of eastern England. Furtive, alert and slightly concerned at staying in one spot for even a short period of time, he suggested we get into a car and drive.

So through the gloom and the mist we drove across the flatlands of East Anglia; past small villages and tiny hamlets which still twinkled with multi-coloured Christmas lights.

All the while, the hyperactive west Belfast man and former spy outlined his plans for a new battle - this time against the very people tasked with his ongoing personal protection.

McGartland, who escaped two IRA murder-bids, including interrogation at the hands of the Provisionals' notorious 'Head Hunters', is taking on MI5.

He alleges that, when his personal security was handed over from British police forces to the Security Service, he was put at unnecessary risk and received no treatment for Post Traumatic Stress Disorder (PTSD).

As regards the latter, McGartland certainly has good grounds to claim he suffers from PTSD. He was shot seven times in the second attempt to kill near his then home in Whitley Bay in 1999.

He is still recovering from the trauma of being beaten and submerged in water eight years earlier in a flat on the Twinbrook estate prior, presumably, to his planned 'execution' for informing.

Moreover, while the Provisional IRA campaign is over and the organisation has marched into history, the recurring IRAs of the Continuity and Real variety all pose a lifetime threat to the spy who infiltrated the Provos.

The former petty criminal was unique among those within the IRA who betrayed its secrets. The RUC persuaded him to infiltrate the IRA in west Belfast in the late-1980s.

The overwhelming majority of agents were already IRA members when they were recruited to work for Special Branch, MI5 or military intelligence, but McGartland was different: he was an outsider encouraged to go in and spy on the most dangerous terror group in the Western World.

During his five-year career in the Provisionals, McGartland betrayed dozens of IRA operations and is estimated to have saved up to 50 lives - hence the title of his first memoir Fifty Dead Men Walking. McGartland - 'Agent Carol' - was regarded as one of Special Branch's best spies, but was compromised and unmasked as a traitor.

McGartland was taken to a flat by an IRA internal security unit who tortured him by plunging his head into a filled-up bath. When an Army foot-patrol passed by the flat, McGartland's captors got nervous and untied their prisoner in case the soldiers raided the premises.

Once free, McGartland hurled himself through a window and fell two storeys onto the ground, landing on his head. After an ambulance was called, McGartland was taken to hospital and gave a nurse the contact of his RUC handler, known as 'Felix' (played by Ben Kingsley in the film of the agent's life).

Ever since, he has been, in the words of the title of his second book, a Dead Man Running, living the remainder of his life in the knowledge that he could be shot and killed at any time.

Indeed, in 2008, the Real IRA issued a statement naming several informers from the Troubles they still regarded as targets.

McGartland was high up on their list.

"I had two near-death experiences at the hands of the IRA. I was shot nine times in their second bid to kill me. Yet I received no proper treatment for PTSD, or any rehabilitation," he complained through our long journey into the night.

"After the shooting in Whitley Bay, there was a big gap where I never got any treatment for nearly five years and my condition got worse.

"I intend to produce medical evidence and psychiatric reports in court to prove that."

As well as alleging he received little or no treatment for PTSD after 2001, when MI5 assumed full responsibility for the state agent, McGartland claims officers from the Security Service made a number of serious security gaffes that exposed where he was living in hiding. Although he declines to go into the details of how his personal security was compromised, the ex-agent says his legal team are preparing a case file which will be used in a landmark civil action against MI5 for negligence later this year.

In doing so, the agent-turned-author may be about to open the legal floodgates. Other agents - including a Russian national who worked for the Secret Intelligence Service, MI6 - have recently asked to be put in touch with McGartland and his lawyers. The Russian former agent also believes he was let down by MI6 after years of service to British Intelligence. The principal reason McGartland cites for suffering alleged neglect is that he broke his silence by detailing his incredible story in the book Fifty Dead Men Walking.

Whatever the veracity of that claim, the planned case against MI5 could, as a by-product, conjure up some fascinating ghosts from the past.

During our short journey together, McGartland indicated that he will, once again, name the names of the men who kidnapped and interrogated him and ask pointed questions as to why none of them was ever questioned about his abduction.

In addition, he will again raise the spectre of the disjunction between various branches of the security forces and the possibility that other state agents inside the IRA were protected from scrutiny - even though it almost led to him losing his life in 1991.

The potential therefore exists, via the McGartland case, for yet another fascinating insight into the dirty and shadowy war played out between paramilitaries and the security forces in the complex hinterland where expediency and source-protection often triumphed over morality and loyalty.