Would that really work, Colm? I'm not sure it would, albeit based on not much thought. Basically, while E=mc^2 implies a fair amount of energy on the face of it, electrons and positrons are so pathetically small that you would a lot of them to make a bomb of any sort of power, and then you'd have to be able to trap a lot of them too. Quick back-of-the-envelope calculation, possibly wrong by a couple of zeroes, you would need about 100 trillion or so positron-electron pairs just to give one joule's worth of energy. That's basically unfeasible.
Antimatter bombs sound cool but they are, for now at least, unfeasible.