These are questions we really shouldn't have to ask, but previous cases have shown the British security services applying vast resources to threats which are scientifically improbable, and then resolutely insisting that their actions were entirely justified and appropriate. Police sources still claim that the ricin plot was a serious threat (Charles Clarke, Home Secretary at the time of the trial, avoided admitting ricin hadn't been found by suggesting in a parliamentary answer that the presence of ricin in a castor bean somehow 'counted'), and also still claim (but generally only when asked directly) to be searching for the improbable Forest Gate bomb (although 'sources' have now downgraded this to a cyanide weapon)*.
Given that the defendants in the red mercury case were talking to a fake sheikh in Brent Cross Holiday Inn about getting hold of the mythical substance, as opposed to just trying to buy it on the open market, one might reasonably make some inferences about their motivation.
They appear to have thought a) That it was a chemical used to wash stained banknotes; b) A highly-prized medicine; c) A way to make some money; or d) Worth up to $300,000 (some useful background here, spoiled by a final bit of BBC-style fence-sitting). The surveillance seemed to indicate only that it was Mahmood who'd been stressing the 'dangerous' nature of red mercury, that nobody was sure what it was, and that one of the defendants at least had been looking it up on the Internet.
So, if the terror squad became involved in Mahmood's operation because they genuinely believed in the existence of red mercury, we should worry. If on the other hand they didn't believe it existed but pursued the case on the basis that people attempting to obtain terror weapons, non-existent or not, should be caught, we should still worry, but for different reasons.
The security services claim that in the face of today's terrorist threat it is important to move early, before the threat has entirely matured and, and often before they have built a case. Problems associated with this approach are covered in some detail here, but note that in this case there was no reason to move early - if the terror squad understood that red mercury does not exist.
If you think some villains are trying to get hold of a non-existent substance in order to build a bomb, then there's no great urgency in trying to stop them. The reverse, in fact - longer surveillance increases the chance of their incriminating themselves, and if you actually sold them something, you might well scoop up would-be bombers, al-Qaeda Mr Bigs, who knows? So moving in without conclusive proof would seem entirely illogical.
It's possible you might have decided, belatedly, that you'd been wasting your time. And one could consider the possibility that having a journalist (well, he says he is) on board tends to set some kind of deadline - these boys need to pay the rent somehow, and can't hang around Holiday Inns forever without a result. What might happen in the (hypothetical, honest...) case of police calling off an investigation because they didn't reckon there was anything there, and a paper running a screamer anyway, about 'these dangerous men' who remain at large?
But we'd be the last to suggest that the popular press defined police schedules, or indeed that the police would let it. Scotland Yard defended the investigation, and said that it would not rule out working with the NotW again, while a Crown Prosecution Service spokeswoman said it was right to bring the case, and that the evidence had been credible. No future intent to work more closely with Porton Down, or even Wikipedia, seems to have been expressed. ®
* Doomweapon update: From the would-be terrorist's point of view, cyanide has the virtue of being more readily available than the more esoteric substances that make it into the headlines, because it's used in volume in some industrial processes. Weaponising it, however, is a challenge, as the explosion intended to vapourise and disperse it will tend to ignite it instead.
This happened in the case of the first World Trade Center bomb, where the cyanide compound added had no measurable effect. An alternative method is claimed (in Ron Suskind's pants-scarer The One Percent Doctrine) to have been planned for alleged attacks on the New York subway in 2003. The design, which Suskind says was found on a laptop in Saudi Arabia, envisaged two chambers, one containing sodium cyanide and one hydrochloric acid. Break seal between the two (with, for example, a small detonation), the chemicals mix, and the result begins to disperse. "In the world of terrorist weaponry," Suskind tells us, "this was the equivalent of splitting the atom."
Which if true suggests that the level of technical expertise in the world of terrorist weaponry is lamentable (or, as the rest of us might view it, hearteningly) low. Rationally, however, we should accept that terrorists know full-well that two glass vessels plus small bang starts to mix chemicals. It does not, however, necessarily have much effect. The removal of the big bang leaves a dispersal problem, and conventional weaponry is as we've seen in Madrid and London far more likely to cause major casualties. There is however a logic to terrorists using relatively low-tech chemical weapons, because of their disproportionately high psychological effect. The more we overstate the potential, the more we panic, and the more attractive they become to terrorists. So go figure... ®