If the stones were in a skip then the ownership is transfered to the waste company otherwise the ownership is with the landowners unless the builders have a contract which states they own the waste from the site.
Well regardless of what has been said to the contrary, a parish councillor has many roles to play in local politics and they often take on responsibilities that are not directly "their job". They often have good contacts at district and county level too.
What has substantially muddied these waters was the removal of the stone from the property without permission of the landowners.
Regardless of whether the landowners own the stone it is always something that will be brought up in discussion and something that most people who can help with preservation will not want to be associated with.
A parish councillor may have contacts but... there are certain things which are most certainly not 'their job' and they should and must be very clear about what they can and should not do within that role, and how they should operate.
Enforcement investigation is legally the role of the enforcement officer at district council level.