Leak in extended/excavated basement

We recently purchased a condo duplex in a newly renovated townhouse, consisting of the first floor and a finished basement (sub-grade and excavated deeper for ceiling height). Since we moved in 8 months ago, we’ve had a number of flooding/leakage problems. Many have been related to our neighbors’ poor maintenance of their building, but there is also the argument that had our basement not been extended deeper than theirs, we would probably not be taking on their water. At present, we’ve had a slow, steady leak for a month and a half in one of our basement corners and have been unable to find the source of it.

Has anyone else had experience with this situation and its legal and/or practical redress? Any advice would be appreciated.