As far as I can see, having the directors sign off on an audit,...

As far as I can see, having the directors sign off on an audit, at least in the spirit of the law as it now stands, is to ensure that:
directors are aware of and endorse the selection of the auditors;
directors are aware of and endorse the fact that the audit has been completed, that audit documents/report have been received from the auditors, that any laws regarding the commissioning of auditors has been followed, that all the relevant documents have been made available to the auditors and that the auditors have not been obstructed or misled by the directors.

Whether or not the letter of the law follows my assumption is another question.

But, having satisfied the above requirements, the directors are not, or should not be, responsible for the audit report. If an error is made by the auditor then the question is: were the auditors in receipt of sufficient information or had access to sufficient information to make a correct determination (to not make the error).

If not, and if it was misinformation or withholding of information on the part of the directors, then the directors rightfully should be held be responsible.

That is my take. In other words, directors should not be expected to have auditing skills at or above their own auditors. If you hire electricians and they do work for you, is it your fault if, after inspecting their work, you fail to see a fault? Well, you're not an electrician. It is only your responsibility not to hire an amateur, to allow them to inspect all your wiring and report to them any faults that you are aware of.

Assuming that PWC have an accounting and auditing competency greater than the directors, then I don't see how the directors could be liable for an error made by the auditors.

Robert