I'm not sure that it would be sole evidence, in much the same way as one call to Childline isn't enough to lock someone up, but it is used to raise questions and promt an investigation. The question isn't really whether the evidence would lead to a prosecution, but an interesting fact of the law that if you know or suspect child abuse in England and Wales but do not report it to the authorities, you can be prosecuted.
of course, if nobody actually listens to the voice recordings and doesn't find the "Please make Daddy stop." messages, then it could be a different kettle of fish. The legal precedent would be interesting, though.