How Did Mossad Know the London Bombings Were Going to Happen?

"al Qaeda sleeper cells" are said to be totally independent, highly organised and shrouded in secrecy:

"The way these folks work, they can go the logistics planning, to get the equipment they need and do the surveillance. Al Qaeda-trained people do a tremendous amount of surveillance to find the right targets, the right time of day or whatever. But then, you can put all those plans and equipment on the shelf, and then with one phone call, in as little as 24 hours, execute those plans." [CBS News]

Four people were supposedly involved in the London bombings, no-one else. If this four man "sleeper cell" had been infiltrated then the bombers should not have made it into London. Similarly, if their communications had been intercepted they should not have made it into London.

They did make it into London.

Common sense says suicide bombers would not openly discuss their mission whilst in London, so how did Mossad know of the impending attacks?

...the Mossad office in London received advance notice about the attacks, but only six minutes before the first blast... [yetnews]

Also, it was thought the explosions were due to power surges until 10:00 a.m:

8.51AM: Seven people are killed in an explosion on a Circle or Central line train 100 yards from Liverpool Street Underground station.

9.33AM: All London Underground services are suspended - the network says it as the result of a power fault across the network.

10.00AM: The National Grid, which supplies power to the Underground, confirms there have been no problems with its system. [manchesteronline]

Why didn't Mossad inform everyone that the explosions were being caused by bombs?