I'd be surprised if anyone could find the original article online, as any publisher who repeats it is also at risk of a libel action. I saw it, and it was clearly, obviously actionable. If Singh didn't know that, he's not really very competent as a 'writer' and the subs at the Guardian took a mad risk.
Singh is entitled to disapprove of chiropractic - I'm not too keen on it myself. He's entitled to discuss research into chiropractic. But he's not entitled to make accusations that are clearly actionable, no more than anyone else is. Or, at least, if he chooses to, he shouldn't be surprised if he's sued.
ooj, that's not the way libel works, as any journalist or writer who is published in the papers knows or should know full well. Apart from Singh, apparently. In libel law, the burden of proof is reversed - the person complained of has to prove their claims were true, or honestly held, or reasonable.
So Singh is a. not a doctor and b. acted in this case as if he doesn't know the very basic rules of journalism.