damp proofing internal wall in 1880s sandstone house

Difficult to define the problem without seeing it but any type of injection or sealing may cause issues with the construction due to the age of the property as this can cause the mortar to crumble as too dry and cracks appearing, testing the damp to find out if it is ground water and some investigation on the construction of the new build may help in finding out the issue, if it is ground water then it could be as simple lifting the skirting which will allow the wall to breath and the damp to escape naturally, a damp survey would be needed with the proper equipment ie upto date meters not pinned, thermal image camera and equipment to test for nitrates etc, Property surveyors will not have this equipment so dont waste you money on them as they will direct you to a damp proofing company.A property of this age will have inherent damp which it has been happy with for many many years and with small adjustments wont be an issue, Hope this helps.