Wildlife pond creation
There are always work arounds. The pond could be slightly raised if roots are an issue. Regular cleaning/ dredging or even a net over the surface can help with leaves (although would not be my choice). But oxygenating plants are a must. Just a few will quite quickly multiply into plenty for a small pond. I tend to agree with Lyn about using liner rather than pre formed, much the easier option in an awkward site.
Really hope you get to attract your wildlife. Gardens in urban/ semi urban areas are exceptionally valuable.