"A settled calm is the first quality you notice about Alice Sheridan.

This is a woman who for years has suffered a catalogue of illnesses, spondylitis and two battles against breast cancer. But someone, too, who has never allowed composure to be hijacked by pain. Now, however, there is pain of a different order to be managed and absorbed. “When I saw my son go to prison today” says the mother of Tommy Sheridan, “I said : ‘There goes a man who can’t be bought.’ ”

That sentiment is an article of faith for Mrs Sheridan, and I first heard her articulate it when we talked at her home before Lord Bracadale sentenced Sheridan to three years in Barlinnie. Yesterday, I heard it again, just audible above the sharp click of handcuffs on Sheridan’s wrists as he was led from the dock; Alice’s incantation directed, one felt, as much at the Almighty as to the courts of justice. But uttered with dignified, maternal resolve rather than belligerence.

As we conversed in the sitting room of her small flat off Paisley Road West in Glasgow, she couldn’t entirely disguise the chronic discomfort from the spondylitis and fibromyalgia that cause her to use a walking frame outside. “But I’ve never used the frame when going to the court because from the start I was determined to walk that path to the entrance of the High Court , without it.” If emotional stress has aggravated her physical condition, it hasn’t dented her spirit.

“I believe that Tommy is the victim of the biggest, most insidious conspiracy since the Dreyfus affair,” she says, a reference to the political scandal that divided France in the 1890s and early-1900s when a French artillery officer of Jewish descent was wrongly accused of treason.

That’s quite a claim and one which, beyond the cauldron of militant politics, might seem ridiculously bombastic. Yet Alice is adamant her son’s fate has not been about politics but civil liberties. “This has been going on for 15 years from that moment when (Rupert) Murdoch first put out the word to ‘get that two-bit, Commie b****** at all costs.’ Well, Tommy has never been a Communist through choice. He’s not a b******. I’ve got his birth lines to prove that. And he’s never been two-bit. So, this is a big, big conspiracy, and I believe that everyone who spoke against him lied.”

Beneath the dignified exterior, does she feel rage? “Yes, but I used to be a social worker so I know rage has to be used constructively, so I’ll do that by helping the Defend Tommy Sheridan campaign as best I can.”

In retrospect, was it a mistake for Sheridan to sack his defence team twice and represent himself? “No, no, no. I’ve absolutely no regrets about that and numerous QCs have approached me since the trial to say they couldn’t have conducted the defence as well as Tommy did.

“Most of them are experienced in dealing with everyday murder but, you see, no matter how good they might be, they’re not well versed in conspiracy cases, and, as I keep stressing, this is a conspiracy.”

How does a mother endure the squalid things Alice Sheridan heard against her son in Court Four?

“As a family we had heard it all during the libel trial in 2006. So, by this time, we were anaesthetised to the worst of what was said. What was new to us was the video. But it’s not Tommy in that video. Tommy doesn’t smoke; the person in that video is smoking. And Tommy doesn’t use that kind of language. So, that was a stitch-up between the News of the World and the Scottish Socialist Party.”

Since the perjury trial, one person who was called to the witness stand has experienced his own ignominious fall from public life. Andy Coulson was editor of the News of the World when the paper paid £200,000 for that video footage.

The Metropolitan Police has finally reopened its investigations into the phone hacking scandal and Coulson has resigned as David Cameron’s director of communications, while still denying prior knowledge of both the proven and alleged offences. For the Sheridan circle, his exit from Downing Street must seem like some vindication at least.

“Yes, because Tommy brought a lot of that stuff out at the trial. I’d watch him stand up and go to the lectern in the dock, and I’d think: ‘God love you, son, how broad are your shoulders because you’ve taken on the manky Murdoch empire.’

“That fills me with enormous pride, and, although I know people can be very disparaging towards Tommy, there are thousands who love him.”

Alice Sheridan, born in 1938, was raised in Govan and married Tommy Sheridan Sr, an iron moulder to trade, when she was 19. The couple separated but they remain on amicable terms.

“Tommy the elder isn’t as outspoken as I am. He’s more political with a small p while I’ve always been more of an activist,” she says. In her middle years Alice was among those instrumental in establishing Scottish Women’s Aid, the refuge movement against domestic violence.

A feel for social justice began when Alice attended St Constantine’s School in Govan. Her parental home was modest and comfortable but among her fellow pupils were children from such impoverished circumstances they were dressed in the charity uniform of parish suits. “That’s what their clothes were called, and the cloth was so coarse it chafed their skin. It was terribly sad because it stigmatised them. And for folk of my generation that stays with them.”

When the Sheridans’ three children were young, Tommy Sr ran Pollok United Boys’ Club, a football club that his son credits with preventing many boys, himself included, from turning bad. “ It was strictly run, with training sessions several nights a week” reflects Alice. “So my house was like a Chinese laundry with all the strips I was forever washing.”

In adversity religious belief has always been Alice Sheridan’s support system. Just months before the perjury trial, she underwent a lumpectomy for the recurring cancer. But she missed only one day’s attendance during those 12 weeks at the High Court, and always in her handbag there were a small phial of Lourdes water and a rosary, the talismans of life-long Catholicism. It is worth mentioning because it explains something of Alice Sheridan’s understanding, since childhood, of the purpose of faith.

“I brought up my children, Lynn, Carol and Tommy, to believe that if you see injustice, you must challenge it, whatever the cost to yourself. If you don’t, you pay an even greater price, that of not being able to live with yourself.”

Faith has seen her through much adversity and it will be her coping mechanism now. “Some people see faith as a cop-out, an excuse not to do anything. But that’s a mockery of faith. I’m not saying people of no faith don’t work for the common good. Actually most of the atheists I’ve met are more Christian-like than many self-professed Christians.”

But what of Tommy, raised a Catholic and now an avowed atheist? In jail, what will be his coping mechanism? “He’ll cope because of his belief in socialism, justice and equality, and the knowledge that he’ll always have the love of his family and friends.”

She remembers that, when he was about 19, Tommy told her he no longer believed in God. “I replied: ‘That’s all right son, because he believes in you.’ ”

She’ll keep offering up prayers for him, and, when you ask if she has a favourite saint, Alice reflects that she has millions. In matters of heavenly intercession, she laughs, you’ve got to keep in with everybody."