This post at Captain's Quarters — suggesting that the bombers were implementing the plan which had been previously foiled with the arrest of Naeem Noor Khan — reinforces something I had in mind while writing the piece below, but didn't actually state.
It strikes me that the bombers' tactics were formulated to be maximally simple and, above all else, to minimise the chances of being interfered with.
Once the explosives had been made and made into bombs, the operation was carried out in as few hours as possible. Drive down from Leeds, dump the car in Luton, get on a train to King's Cross, spread out a little and blow up. There are many refinements that might have increased the impact or reduced the amount of intelligence available afterwards, — getting on tube trains coming in to King's Cross rather than away from it, heading for a high-profile target, say Westminster or Canary Wharf, staggering the explosions over hours or even days, or simply hiding the car somewhere rather than leaving it in the station car park, but if one of the four was already suspected by the security services from a year ago, then any delay at all might have provided the opportunity for the authorities to spot what was happening. As it was, they might have been followed by police the whole way, and would quite possibly have still managed to cause as much damage as they did. (Not that I'm suggesting they were followed, but they might have feared they could be under surveillance).
This suggested to me that the plot was very small in terms of personnel (which is somewhat contradicted by this new report), and, along with other things, that the terrorists are weak, only able with the greatest difficulty and after several failed attempts to achieve any kind of successful operation, and that a suicide mission not just for the bombers but for their cell as a whole, which is being rapidly rolled up.