nUnit - ignore GAC copy of a DLL

Question

I am using a nUnit to, well, unit test an assembly.

The assembly is in my project output dir (\bin\debug) and is loaded into nUnit (Assemblies > Add Assembly) from this location.

However an older version is also in the GAC and nUnit is picking this one up instead.

I can of course remove the old version and re-install to the GAC upon build but this takes some time - any way to force nUnit (or more likely the .NET framework) to pick up the version from the bin\debug dir?