“How can a local authority fulfil these duties in the problematic cases with which we are concerned if they cannot obtain authorisation from the High Court to place the child in the only placement that is available, and with the ability to impose such restrictions as are required on the child’s liberty? It is such imperative considerations of necessity that have led me to conclude that the inherent jurisdiction must be available in these cases. There is presently no alternative that will safeguard the children who require its protection.”