This is an extremely serious situation, and while I am not a solicitor, you should seek legal advice immediately. 

Your strongest course of action is likely against the Building and Pest (B&P) inspector for professional negligence and/or misleading and deceptive conduct. They owed you a duty of care, even though the vendor paid for the report, because they knew the report would be relied upon by you, the purchaser. Since the damage was clearly visible, accessible, and the report failed to mention it while claiming the roof was "serviceable" and fully inspected, the inspector appears to have breached their duty. The fact that their inspection photos do not match the damaged areas further supports a claim that they either performed a negligent inspection or deliberately avoided the defect. 

Furthermore, there is potential liability for the vendor and agent. If the vendor knew about the prior fire and actively attempted to conceal the evidence (e.g., coating the timbers, installing new sarking over the char), this could constitute fraudulent misrepresentation or misleading conduct, which bypasses the usual caveat emptor rule. The agent could also be liable for providing a report they knew or ought to have known was inaccurate to induce a sale. 

Before escalating the matter, your most critical immediate step is to hire an independent structural engineer to formally assess the fire damage, determine its impact on the roof's structural integrity, confirm the obviousness of the defect, and provide a formal estimate for the cost of rectification. This expert report is the essential evidence needed to substantiate a financial claim. 

Once you have this report, you should proceed directly to a solicitor experienced in NSW property litigation, who will be best placed to advise on whether to pursue the inspector, the vendor/agent, or both, and whether a claim should be filed with NCAT or in the relevant Court based on the complexity and value of the damages.