Hi Rob,
> Your problem looks like the createIfNecessary method on FileSystemRepositoryAccessor is failing. 
> It turns out there is a FileSystemRepositoryAccessorTest but it isn'r fully testing the creation part
I would recommend that you fix the TestCase to fully test this situation.  This would help all of us.
    What ! Me fix ModSqueak ? You must be kidding. Anyway, encouraged by your hints and recomendation, I just put an order for 3 books. The bookshop promised to deliver the first within 5 weeks, the second and third within 5 what (my cellphone went nut here). The books in order are 'Smalltalk-80: The Language', 'Squeak: The Image' and 'ModSqueak: The Framework'.
    I will try to fix the TestCase when I got them all  ;-). In the mean time ...
> I should have been clearer, this fix was for running ModSqueak on SWT0.6.
    Do you think, instead of '0.1' (for SWT0.5) should we use '0.2' (for SWT0.6) instead:
        | url |
        url := (RepositoryUrl fromString: 'repos://SqueakBase/SUnitConfig/0.2').
        url resolve build.
        url := (RepositoryUrl fromString: 'repos://SqueakBase/RefactorAndLintConfig/0.2').
        url resolve build
    Have you tried this ?