The recent attempted bombings in London and Glasgow have highlighted the fact that Britain remains a prime target for al-Qaeda. Outside Iraq and Afghanistan, Britain is al-Qaeda's most popular target, having faced more attempted attacks than any other country.
Leaving aside various ineffectual plots, fundraising and propaganda efforts, the so-called Doctors' Plot was at least the fourth serious attempt at inflicting mass civilian casualties by the supporters of Osama bin Laden. There are historical, social and political reasons why this should be the case as is demonstrated by the four main bombing conspiracies.
The 7/7 bombers and those convicted in the Operation Crevice inquiry for conspiring to bomb the Bluewater Shopping Centre were primarily made up of young British-born Muslims whose families had emigrated to Britain many years ago from Kashmir in northern Pakistan. The importance of Kashmir to this group of al-Qaeda sympathisers - many of them associated with the now-banned al-Muhajiroun organisation - was underlined by the arrest of Dhiren Bharot in 2004 - now serving a 30-year prison sentence.
Bharot first travelled to Kashmir in the mid-1990s and wrote a book - The Army of Madihah in Kashmir - recalling his own experiences fighting the Indian Army. "Initally he fought in Indian-occupied Kashmir against the Hindu oppressors for a while, before returning. One of the few not martyred from his group...", the blurb on the book's cover reads.
Bharot's book found an eager audience amongst aspiring young jihadis in Britain. Others were to follow in his footsteps, including Mohammed Siddique Khan, leader of the 7/7 bombers, Omar Khyam of the Crevice plotters and Mohammed Junaid Babar, who was a member of al-Muhajiroun in New York, but connected to the Crevice plotters.
With more than 400,000 people travelling from Britain to Pakistan every year it has been relatively easy for fired-up young zealots to visit Kashmir or the Tribal Areas of Pakistan, get military training and return to await a call. The 21/7 bombers came from an entirely different background. All four men convicted in July arrived in Britain from the Horn of Africa as child refugees.
In Britain they were radicalised by the other major pole of attraction for young Muslims - the Finsbury Park mosque and its now imprisoned leader, Egyptian-born cleric Abu Hamza al-Masri. Unlike those of Pakistani origin, this cell had more of a background in petty crime and street gangs. Much of their youth had been spent clubbing and taking drugs. Formally uneducated, they lived primarily on state handouts and spent much of their time at the Finsbury Park mosque.
Their leader, Mukhtar Ibrahim, who was initially radicalised while serving a prison sentence, travelled to Pakistan in 2004 and is alleged to have spent time with Mohammed Siddique Khan at a training camp where both men learned to make the hydrogen peroxide bombs they eventually used in London. The judge at his trial said he had little doubt that the 7/7 and 21/7 attacks were coordinated, probably by senior al-Qaeda figure Abd al-Hadi al-Iraqi who is now in Guantanamo.
The Doctors' Plot appears to have involved yet another kind of group. All the members of this group were recent arrivals in the UK, of Indian and Arab background. Highly-educated professonals, it is less likely that any of them have spent time in jihadi training camps in Pakistan.
Their chosen method of attack was crude and ineffective. We can see from these bare facts that al-Qaeda's leadership in Pakistan and its main franchise in Iraq have had a close relationship with the attacks in Britain. They give the lie to the old saw from Omar Bakri Mohammed, the leader of al-Muhaijroun, who used to argue, before he fled to Lebanon, that Muslims in Britain lived under a 'Covenant of Security' that guaranteed no attacks in exchange for the right to propagandise their beliefs.
The four attacks also demonstrate that al-Qaeda will adapt its methods to suit particular conditions, using different social groups and varying its techniques. The real question now is not so much whether it can continue to find willing recruits to carry out further atrocities in Britain - in the short term the answer is yes - but where are we on the curve? Are we likely to see a growing number of attacks or will they begin to diminish? Looked at purely from the standpoint of effectiveness, most of the attacks in Britain have been dismal failures. Only the 7/7 bombers managed to carry out their missions as planned.
The rest have resulted in capture and long prison sentences. More than 20 would-be 'martyrs' are now in prison serving life sentences where they are unlikely to provide inspiration to a new generation. What is remarkable about all four conspiracies is that significant moments in all the plots were captured on surveillance cameras. This is unprecedented. Compare this situation to the 30-year conflict in Northern Ireland where surveillance footage of IRA volunteers in action is almost unknown. Like it or not, Britain's thousands of security cameras now mean that anonymity for bombers is almost impossible.
And while the Security Service, MI5, has rightly been criticised for its failure to stop several of these attacks, it is also true that very few of the conspirators were completely unknown to them. This demonstrates that the watchers are looking in the right places.