  A complete and simple invariant classification of the conformally flat pure
radiation metrics with a negative cosmological constant that were obtained by
integration using the generalised invariant formalism is presented. We show
equivalence between these metrics and the corresponding type O subclass of the
more general spacetime studied by Siklos. The classification procedure
indicates that the metrics possess a one degree of null isotropy freedom which
has very interesting repercussions in the symmetry analysis. The Killing and
homothetic vector analysis in GHP formalism is then generalised to this case
were there is only one null direction defined geometrically. We determine the
existing Killing vectors for the different subclasses that arise in the
classification and compare these results to those obtained in the symmetry
analysis performed by Siklos for a larger class of metrics with Ricci tensor
representing a pure radiation field and a negative cosmological constant. It is
also shown that there are no homothetic Killing vectors present.
