There are several considerations which arise as a consequence of the circumstance described.
Some might be considered important, some might be considered relevant and then the whole package might also be described as irrelevant since the absence of, or failure to abide by, existing policy has very little, if any, consequence anyway.
First of all, it is a red herring to consider whether the PC has any liability in any regard. It doesn't. It doesn't own the land, it didn't engage the contractor and it hasn't (yet) made any financial contribution. So there is no liability.
The work has already been done - was there an accident? If no, why would a public liability exist after the event? It wouldn't. The PC should have no interest in whether any, or to what financial limit, the contractor might hold public liability insurance. It is completely irrelevant since the contractor was not engaged by the PC.
Can you make a grant? Do you have a grant policy? If you did, and it followed the normal format, it would probably preclude approval / payment after the event, it would probably preclude items of general / recurring maintenance and it would require the benefit derived to be loosely commensurate with the cost of the grant. There could also be minor details like requiring PR attributed to the PC for grant funding.
But if you don't have a s137 policy you can neither follow, nor contravene it.
You must however ensure you are lawfully empowered to spend the public money - how will you ensure that unless there is a policy or a legislative framework? How will you demonstrate VfM and diligent management of public money with no policy?
So, whereas it 'could' result in criticism from the internal audit, it would make no difference if it did since there really are such limited and pointless sanctions as to make the whole audit process meaningless.
Google s137 policy, cut and paste into your own template, validate at next PC meeting.
You'll probably find most (½ decent) policies require payment to be authorised prior to expenditure / expence being incurred.