Regardless of whether this latest investigation yields results, the threat remains substantial. Officially, it is classified as "Severe" – which means an attack of some kind is highly likely. It is tempting to believe that this is overstating the case. We have not had a successful attack here since the London bombs of 7 July 2005, in which 52 people died.
Some critics of the Government question whether the terror threat is exaggerated for political expediency. Recent attacks in the UK, such as the failed bombings in London and at Glasgow airport in 2007, have attracted ridicule for their amateurish execution – as did the failed bombing of an Exeter restaurant last year.
Certainly there are signs that the police and MI5 have a much firmer grip on the problem of home-grown terror. After years of neglecting the rise of ideological extremism in Britain, which arrived with the radical clerics in the early 1990s, MI5 has identified more than 2,000 people it considers a threat, many of whom will be under surveillance. With the help of the Americans, who run the giant listening and data processing "factory" at Forte Meade, near Washington, billions of communications are sifted each day, patterns of behaviour are constructed, clues are unearthed.
None of this, however, detracts from the fact that al-Qa'ida would dearly love to deliver another terrorist spectacular such as 7/7 or 9/11.
In the past, successful terrorist plots in Britain have relied on British citizens, who have been radicalised here, going to Pakistan for final training. This is now far harder and a threat is emerging from the increasingly ungovernable territory between Pakistan and Afghanistan. There is no shortage of willing, ideologically indoctrinated recruits from these areas, and they could be sent to Britain to act as sleeper agents, waiting for the right time to attack. Whether the counter-terrorism operation in the North-west is the first UK example of this approach remains to be seen. It is entirely possible that these men are innocent. But a strategy to send recruits from Pakistan is a distinct possibility, a strategy which has close parallels with al-Qa'ida's biggest success – the 11 September attacks of 2001.
The writer is the terrorism specialist for BBC2's Newsnight