Code peer review tool

I’m looking for a good peer code review tool where I can choose which users are most appropriate for my review (although others can self join), and keeps track of changes which essentially update the code in the review (in other words I don’t want obsolete code to remain in the review, rather be replaced by the later version).

Essentially think along the lines of Crucible/Fisheye but 100% free and hostable in house.

I know of CodeBrag which is ok for small teams but doesn’t seem to provide the sort of appropriate peer functionality I am looking for.

I also know of Gerrit but this seems to contain it’s own central repo where as I already have a fully functioning central repository.

The other slight hinderance is the current use of SVN as a versioning tool. CodeBrag thankfully has git-svn support but Gerrit seems pure git (although I assume git-svn could be shoehorned in to the flow).