Issue with environmental searches

Lou837

I’m in the process of selling my house. I bought it 5 years ago without any issues. However, our buyers environmental search, says that we have an unidentified tank which was in the area of our living room in year 1890. The company that did the search has said that there is a “high risk of ground contamination” our buyers are now at risk of losing their mortgage offer. To add insult to injury the same company did a survey 9 years ago and gave the house the all clear. The house is a converted barn and has swapped hands several times since the conversion 32 years ago. I cannot find any information about a tank, have no idea whether it was above or below ground, or what it might have contained. I have contacted building control and they have no information about a tank as far as their  records go back, and have concluded that the tank was removed sometime between 1890 and 1991. The tank only shows up on the 1890 OS map.
my questions are;
Who else can I contact to find information?
How do I prove our land isn’t contaminated?
Is there anything that can be done with regards to the company that did the 2014 then the damming 2023 report? 
Thank you if anybody is able to help me with this issue.
kind regards, Louise 

user1977

You prove it isn't contaminated by getting someone to do an intrusive survey rather than a desktop one i.e. take soil samples and test them.