**** BATH BOMB UPDATE ****
It worked! The thing actually fizzed away! Although it did sink to the bottom of the bath like a lead balloon! Apparently the cornstarch (which I didn't use as I didn't know what it was) is what helps the bombs to float. It was so big and heavy though that I doubt anything short of an industrial buoy would have kept it on top of the water!!
The kids liked it though, and were suitably impressed that I'd made it myself. Needless to say that want to have a go now so it looks like another shopping trip is in order!