This is a pariliamentary briefing paper that Nafeez Ahmed wrote regarding the involvement of the spooks in the 7/7 network. Some of this information was redacted from his book "The London Bombing" (also worth a read) but this .pdf contains all of the material, uncensored.
After reading this document there should be no doubt that 7/7 was, at the VERY LEAST, an example of LIHOP (Let It Happen On Purpose), i.e. "they" knew about it and made sure people looked the other way, the path was cleared as much as possible. At the very least. Anyway, highly recommended reading on 7/7.