Sorted this issue out. I was trying to set the file as a folder.
For anyone out there with similar issues, check you're not making the same mistake and this code works .
I can now map from the specifed TFS server path to the specified local path.
Workspace workspace = tfServer.CreateWorkspace("TempDevWorkspace", tfServer.AuthorizedUser);
string topDir = null;
string localDir = @"c:\TempDevWorkspaceFolder";
workspace.Map(tfsItem.VcDevFolder, localDir);
workspace.Get();
string directory = Path.Combine(workspace.Folders[0].LocalItem, null);