The certification body told us that the reduction is possible as they are also carrying out an assessment to PAS99 at the same time. ie if we're not going for PAS99, there's no reduction. I hope that's clearer.
I am puzzled by the CB statement. PAS 99 provides for an integration of management (sub)systems framework. It states:
Compliance with this PAS does not in itself ensure conformity with any other management system standards or specifications. The particular requirements of each management system standard will still need to be addressed and satisfied if certification, where sought, is to be achieved.
So, if your client will be seeking certification to ISO 9001, ISO 14001 and OHSAS 18001, those standards will still be used as the source of requirements.
The reduction in audit time (and associated certification costs) come from the realization that, if the organization indeed has a truly integrated system, one does not need to test the common processes, such as, for example, document control
separately for quality, environmental and OH&S related documents. Similarly, the corrective action process will be tested concurrently for SHEQ issues. Instead of 3 separate interviews with top management, one interview is sufficient to review their involvement and commitment to all 3 disciplines. With that in mind, there are time savings, which are reflected in audit costs. Note, though, that the audit team still has to be competent for all of the applicable disciplines in the scope of the audit.
There have been many organizations who have been certified to multiple standards, in an integrated fashion, who have never heard (nor used) PAS 99.