I was just thinking that. It may be another splinter group from Al Qaeda looking for headlines, and hastily put this together in light of 7/7.
If the explosives were mixed in one batch, the batch could have been bad. Detonators are typically like blanks used in starter pistols to provide concussion that sets off the big explosive.
If the batch was bad, none of the explosives would go off, or would be weak at best (strictly from a technical standpoint, you understand).