I think Kirrkirr can do this. http://www-nlp.stanford.edu/kirrkirr/

Thanks for that, Claire (and thanks to Mr(?) Hat for this post, too!) -- I might have a play with it and see what it can do.

I should mention that I've also recently found this:
http://taporware.mcmaster.ca/~taporware/xmlTools/findtext.shtml
which does do a nice concordance, but you have to know the xml annotation schema to make it work well, and it can't feed the concordance back through my stylesheet, so you get a concordance in xml with tags, which is ugly.