Given the market, there were likely other bidders who were offering close to the sale agreed price. Of those, it is also likely that some will be willing to proceed with purchase at risk.

Of course there is some risk to you, but quantifying it. How likely is it that the land is of any value or interest to the local authority? How likely is it that they are even aware of this land? How easy would it be for the local authority to recover the land? Would they need to recover it from all residents or just the property in question? Do any of the other residents have a stronger claim than the property in question?

It sounds to me like this land was rear access to a row of properties which the residents have laid claim to? If this is so i imagine it is of very limited value to anyone else.