On 15 December 2009, a photograph was taken of Janis Sharp, the mother of Gary McKinnon, and the Liberal Democrat leader, Nick Clegg, outside the Home Office in Westminster. They were there to protest against the extradition of McKinnon, aged 44, to the US on charges of computer fraud. Eight years earlier McKinnon, an Asperger's syndrome sufferer, had hacked repeatedly into Pentagon and Nasa networks.

“They could try him here if they wanted to, so it's up to the government here to do the right thing," Clegg said in an interview that day. "If Gordon Brown really had a moral compass, he would do the right thing and try Gary McKinnon here instead."

Little more than five months later, on 25 May, Clegg, the new Deputy Prime Minister, said of the McKinnon case in a radio interview that "what I haven't got the power to do - neither has the Home Secretary, neither has even the Prime Minister - is to completely reverse and undo certain legal aspects of this. But that, of course, you wouldn't want politicians to do. It's legally very complex."

The truth is out there

Opposition made adopting principled positions simpler. Clegg also stated that his personal view on the case remained unchanged - McKinnon should ideally be tried in a British court. But his equivocation on the law had upset Sharp and, when I visited her recently at her home in Hertfordshire, she wept as she spoke about her son.

“I think we all thought that we had waited until this, the new government; and then we'd done it. They've all made promises," she said, referring to the support offered to the McKinnon campaign not only by Clegg but other senior Lib Dem MPs, as well as David Cameron.

In 2001, McKinnon was living with his girlfriend, Tamsin, in a flat in north London. He was, says Sharp, depressed - without a regular job, and spending most of his time alone in a room, on his computer. It emerged on his arrest in 2002 that, using the name "Solo", he had hacked into 97 US government computers between 2001 and 2002. His actions, according to US officials, caused networks to shut down, damaged computers and incurred costs of $800,000. McKinnon said he had been looking for evidence of UFOs, a childhood interest passed on from his stepfather, Wilson Sharp.

McKinnon was surprised at how easy it was to enter the US networks. There were no firewalls and many government staff did not even have passwords. He left notes as he went, pointing out security deficiencies. One said: "US foreign policy is akin to government-sponsored terrorism these days? It was not a mistake that there was a huge security stand-down on September 11 last year . . . I am SOLO. I will continue to disrupt at the highest levels."

Sharp readily accepts that what McKinnon did was wrong, but denies that he caused damage. (No firm evidence of damage was passed to the Crown Prosecution Service by US officials, and it is not clear any was gathered; a report says their accusations were based on "hearsay".)

On 12 November 2002, six months after his arrest, McKinnon was indicted by the US government on eight counts of computer fraud. He remained at liberty until June 2005, when he was subjected to bail conditions - required to report to a police station every day and forbidden from using a computer. Later that year, the US began extradition proceedings that he and his supporters have been fighting ever since. McKinnon's case now rests with the new Home Secretary, Theresa May. (The Home Office will give no substantial statement on the case, nor any indication when a final decision will be made.) May is specifically considering the medical evidence - the only grounds on which his extradition can be overturned.

According to a number of medical experts, McKinnon is in no state to be transported to the US. His psychiatrist, Professor Jeremy Turk, has said that McKinnon suffers from a "serious major depressive disorder . . . aggravated and complicated by anxiety and panic attacks with multiple psychosomatic symptoms on a background of his having Asperger's syndrome". In November 2009 he said that McKinnon was a suicide risk. "He can't talk about his feelings, he can't open up," Janis said of her son. "I'm frightened that we are going to lose Gary. I'm frightened that he is going to do something. Because there have been close calls in the past."

Sharp had always known her son was different. She recalled how, as a child, he would refuse to travel on a bus and developed obsessions - with the planets and stars. But it never occurred to her that he might be suffering from a severe medical condition (McKinnon wasn't formally diagnosed with Asperger's until August 2008). Now, she is worried about how her son would fare leaving Britain, never mind imprisonment.

“He hates travelling on the Tube," she says. "It's a phobia. He doesn't travel abroad either, he doesn't leave north London. So for him to be put on a plane in chains, it would be torture."

If McKinnon is extradited, he could be given a 70-year sentence in a high-security prison (he would, as an overseas national, be considered a "flight risk", hence imprisonment with violent criminals rather than in an open prison). If tried in the UK - his home country, and where he was living when he committed the crimes he admits to - the sentence would be more lenient. As the human rights lawyer Geoffrey Robertson, QC told me: "The real impropriety of the McKinnon case is the disproportionality in the merciful sentence that he would have received here and the sentence that awaits him."

How have we got to a point where a seriously unwell man might be locked up overseas for the rest of his life for a crime that hurt nobody? The answer goes back to the Extradition Act of 2003. This law implemented the UK's extradition treaty with the US, under which US prosecutors did not need to provide contestable evidence in an extradition case. In the aftermath of the 11 September 2001 attacks, the treaty was geared to help extradite terrorists. It would also remove politicians from the process - a good thing, in principle. However, adequate discretion was not given to judges either, making automatic extraditions worryingly easy to perform. Ultimately, it put politicians in a bind. Over the past eight years, a succession of Labour home secretaries, culminating in Alan Johnson, refused to reconsider McKinnon's case.

Shami Chakrabarti, director of the human rights group Liberty, dismisses Clegg's idea that the law is "complex". "If anything, it's far too simple," she told me. "Too much discretion has been squeezed out of the system, and at this point politicians rather than judges are the only ones who can put it back."

Reforming the extradition treaty would be difficult. But in this specific case, there is a simple political decision to be made - one, says Robertson, that should not be cowed by our "special relationship" with the US. "We were quite happy to upset the US by sending the mass murderer Abdelbaset Ali al-Megrahi back to a hero's welcome in Libya - a false compassion masking a desire to curry commercial favour - and yet we are incapable of true humanity and mercy by sparing a harmless individual whose political offences were driven by an undiagnosed mental disorder."

Instrument of torture

At the Sharps' quiet house, you feel distant from Westminster machinations. We sat surrounded by dogs and musical instruments. Before McKinnon was arrested, the Sharps wrote children's books and played music. McKinnon was also a musician, playing in a band with friends. They showed me a film of him singing against a sci-fi backdrop of moonscapes and UFOs circling night skies.

“Gary can't pick up an instrument any more," says Sharp, "because he says that there's so much emotion he's frightened for it to come

to the surface." Mostly he stays at home, doing very little. His one enthusiasm is cooking - but because of his condition, says his mother, "he has to cut up his mushrooms into an exact amount, and if someone takes a bit he has to do it all over again."

What does the future hold for McKinnon, if the Home Secretary's decision goes in his family's favour? Sharp hopes that he might work with animals - something he has always wanted to do - or return to his music. But first they have to get Gary "back to normal", as she puts it: to rescue him from the "dark place" he now inhabits. And what about her? Her son's case has taken over Sharp's life for the past eight years. It is, she says, all she has thought about every day. She says that if the decision does go their way she will "cry for a year".

McKinnon would then be tried in Britain, but his parents would be close by to support him. He would receive a sentence proportionate to his crime, and would most likely go to an open prison where he could see his psychiatrist regularly. Life for the Sharps could, in some way, return to normal. Someone once said to Janis that at least she didn't have a boring life. "And I said, 'I'd give anything to have a boring life.'"

Sophie Elmhirst is an assistant editor of the NS.