Your situation is complex and clearly unfortunate. Given the circumstances you have highlighted, I anticipate that the principal matter of the uncovering of the lease amendment is a legal issue. It is possible that you would have a valid claim against the vendor for misrepresentation.
As the vendor, having signed the document, was aware of the existence of the lease amendment, she misled you
during the purchase of the lease as she failed to highlight this document during enquiries.
Is the lease amendment a Deed of Variation, or simply a side letter issued by the pubco? If it is the latter, such documents are frequently mislaid. Nonetheless, the vendor should have declared the existence of this document and it may be possible to pursue a claim. It is also surprising that the pubco's solicitors were not aware of the existence of this document, as it should have been referred to within the assignment documentation.
Whilst it may appear that you would have a valid claim for misrepresentation, I am doubtful that this would be an economically sensible course of action to pursue. Although the differential in rental payment is clearly critical to you, it is relatively small. Costs associated with pursuing a claim through the courts are extremely high and I would not be surprised if those costs escalated fairly swiftly to an amount higher than your claim. You have also confirmed that the vendor may be bankrupt and not residing in this country.
The attitude of your pubco is disappointing, but I see this as the only available avenue for you to pursue. I suggest that you continue to seek to enter discussions with the pubco. In particular, I suggest that you obtain a copy of the amendment document and investigate the reasoning behind the alterations. This may provide the opportunity to reduce the rent if the reasoning for the amendment has altered.