In the past, terrorist attacks are often followed by a claim of responsibility, where the perpetrators send in a tape explaining who they are and why they did what they did. “Yes, we’re the Real IRA, not the IRA, and we blew up these passers-by because we believe in a united Ireland”.
But in the case of the London bombs, why would they bother? Everyone knows who did it, and what they are fighting for and why. Claiming would just give the authorities a load of forensic evidence – voice prints etc. The “fifth man” they are pursuing certainly won’t want to stick his head above the parapet.
I wouldn’t be surprised if no-one makes a (credible) claim for responsibility.