Thanks. I started with Marc Edit and you are correct that it is very easy to use. The reason I wanted to do this in XSLT is that is part of a series of other transformations I need to do, and I figured I might as well learn how to do it now. (I am picking up a lot of XSLT skills while messing with these files, and I figure I might as well carry on with doing so.)
But thank you very much for the suggestion - hopefully someone else will be able to use it!