IA report should be the most important as the EA does not check anything unless a formal complaint is raised. However, as DTC has said, it is hit and miss whether the IA does a thorough job or not. Most are happy to tick the boxes and collect the fee - the PC's are also happy with this as it looks like they are doing a wonderful job.
I think the only legal requirement is for PC's to publish the Internal Audit Report page from AGAR and not the fuller report from the IA. Some PC's do publish this too, but a lot don't. If the PC is a smaller authority there is a requirement to publish a full explanation of why any of the IAR boxes have been ticked no, including an explanation of how any weaknesses will be addressed.
Even if there is nothing in law requiring the PC to publish the full IA report, there is nothing to stop you submitting a FOI request for this.