asp.net C# accessing a unc shared folder

I have a webserver and a file cluster which i need to access. I can map the file cluster no problem however when I try to save files to the unc drive i only get errors. in my web config I have the following;

 <appSettings>
    <add key="Docs" value="\xxxxxx-fileclusterfileshare" />
  </appSettings>

<system.web>
    <identity impersonate="true"/>

in the code I have :-

    void SaveFile(HttpPostedFile file)
    {
        // Specify the path to save the uploaded file to.
        string savePath = WebConfigurationManager.AppSettings["Docs"];

on the managed server I have a local account called fileuser and on the domain i have another user called dom/fileuser.

In IIS i have an application pool ruing under dom/filesuer, however still caot save any file to the unc drive.

 Access to the path '\xxxxxx-fileclusterfilesharetest.txt' is denied.

so what am I doing wrong?