Yeah, I had a look on Google as well. Seems that away from the nest they are normally less aggressive than normal wasps ie if you wave your arms around them they will fly off. Thus, seems a shame to kill them given that they are keeping greenfly etc at bay. However, the results of one of the kids kicking a ball into the bush does not bear thinking about. I'm still a bit shocked about finding such an aggressive insect in a suburban garden!