Thanks - I'd missed that sentence about the CVR, Para 19:
19.The CVR recording was found to be of good quality and covered complete flight. However, FDR included the data of the entire flight except after the time when electric power was not available for the FDR. This is as per its designed limitation.
So I guess Airbus and PIA had ?retro?-fitted the 2108 recommended independent, 10 minute-after-main-power-failure battery pack, which I believe had been preferred to RAT-derived power.