It appears the police have now told revealed that they found a number of nailbombs in the car in Luton that was searched 5 days after the 7/7 attack. Surprisingly they did not step up their public protection enough to prevent the attempted attack on 21/7.
Anyway, at least one of the four recently arrested has been confirmed as one of the attempted bombers of 21/7. So im sure we'll soon here what he has to say, if anything at all!
(courtesy of ABC news: One of the 16 unexploded bombs)