SpecFlow wrongly using NUnit

I've just (today) tried SpecFlow for the first time. I'm playing about by creating a new class library in VS2010 Pro and adding a SpecFlow Feature Definition file.

Thing is, the integration doesn't appear to be working properly, with a variety of different errors. I've selected MsTest as the test runner, because I can't be bothered with invoking NUnit (I'd like to use NUnit in the long term but at the moment I just want to get some BDD code working). The generated code files however continue to reference NUnit – which is obviously wrong, since I've just told SpecFlow to run using MsTest. I've done everything I can think of to invoke the code generation again, including creating a brand new class library project with the MsTest option selected in Tools > Options > SpecFlow.

If I leave the test runner field set to 'Auto' and right-click a feature file, then select 'Run SpecFlow Scenarios' I get an error message "Could not find matching test runner".

If I instead change the test runner field to MsTest, I get a different error message on doing the same thing – "Object Reference not set to an instance of an object". I'm not surprised at this one since it's still trying to run NUnit tests even though I've explicitly asked for MsTest, though obviously it shouldn't nullref and present that to the user.

What am I doing wrong? The documentation is not helpful, and as far as I can see, there's no FAQ.

edit #1: I've established that the actual setting I'm looking for is provided using App.Config using the field <unitTestProvider name="MsTest" />. I can see what's happened – the field in the Visual Studio options menu doesn't seem to modify the project you're currently working on. Thing is, this makes it look like that field doesn't do anything at all. I've now persuaded SpecFlow to generate MsTest classes and run using the MSTest runner.

So now the question morphs into a slightly different one: What (if anything) does the Tools > Options > SpecFlow > Test Runner Tool field do?