Likely possibility mentioned by some bomb squaddie who worked in N. Ireland for a while on news radio thingy, was that homemade C4 is relatively easy to make given a couple of petrol based chemicals and various things you can buy from a chemist or local supermarket but for any given detonator to detonate the stuff, you have to practice to get the mix right, which means having to blow things up in your back yard a few times before you've got a bomb that's worth anything. Since Britain doesn't have many places you can test out bombs if you're not the army or a legal arms manufacturer, it's possible the people involved just worked on a guess and got it wrong, which would create a bang from the detonator, loads of white powder and the smell of cordite, as was reported by witnesses present at the bus bombing.