fgaz: what exactly did you put in the file?
for example in my configuration i have
```
nixpkgs.config.packageOverrides = self: with self; 
{
  wine = (wine.override {
    wineRelease = "staging";
    wineBuild   = "wineWow";
  }).overrideDerivation (base: {
    patches = [ (fetchpatch {
      url    = "https://raw.github.com/Tetralet/wine-xinput/master/xinput_quigon.patch";
      sha256 = "0sj3649b2dg8cv32y25ba1fjpxvh7lq7a30npknhg3fxcibfiqak";
    }) ];
  });

}
```
so i write to a default.nix something like
```
{ pkgs ? import <nixpkgs> {} }:

with pkgs;

(wine.override {
    wineRelease = "staging";
    wineBuild   = "wineWow";
}).overrideDerivation (base: {
  patches = [ 
    (fetchpatch {
      url    = "https://raw.github.com/Tetralet/wine-xinput/master/xinput_quigon.patch";
      sha256 = "0sj3649b2dg8cv32y25ba1fjpxvh7lq7a30npknhg3fxcibfiqak";
    }) ];
})
```
and run nix-build