To be honest, I have heard of someone keeping common brown B/N's in a garden pond, slightly north of here (Sheffield) and they have apparantly happily bred in there. As long as the pond is deep enough for them to get out of the way they apparantly seem to be fine. Unfortunatley as we live in a flat, it's not something that I can try out, although we have been threatening to have a go at keeping a few in a unheated tank to see how that goes.