It could have been worse on July 7. From the TimesOnline, with thanks to Jeffrey Imm:
After examining all the possible reasons why the bombers left bomb-making components in the boot of a car at Luton railway station, investigators consider it likely that the four terrorists had intended to build devices with at least 20lb of home-made explosives. The suggestion that the explosives, nails and other components found in the red Fiat had been left there for a follow-on terrorist team has been discounted. One possibility is that their rucksacks were too small for bigger devices.The theory emerged as three Iraqis were questioned by detectives yesterday after being arrested taking photographs near to the venue for a European Union foreign ministers’ meeting next week, near Newport, Gwent. The men were held under the Terrorism Act 2000.
That doesn't quite add up. It's easy to tell in advance how much you can cram into a rucksack. It's quite usual to see people on the tube with much larger bags, suitcases, shopping trollys on wheels, SAS sized bergens even, all during the rush hour so if they wanted a bigger bomb they could have just used a bigger bag. And bomb making incredients are not so very plentiful, and explosives so very easy to make that they can be wasted, even by boys whose mindset is not the same as normal people.
'The suggestion that the explosives, nails and other components found in the red Fiat had been left there for a follow-on terrorist team has been discounted'
But it doesn't explain why that theory had been discounted.