I'm sure this is old hat to you svn experts but I've just figured it out.
If you need to transfer a load of changes from one subversion repository to another (or you have an external developer helping you out on your open source project who has submitted a patch) this is what you need to do:
Generate a Patch File
Move to your "source" working copy and run:
svn diff -r REV > my_patch.txt
This finds all changes since revision number REV and places them in my_patch.txt.
Apply a Patch File
Move to your "target" working copy and run:
patch -p0 < my_patch.txt
This applies all the changes in my_patch.txt to your target working copy. Running svn status then shows all the files that have changed - you can then review the changes before an svn commit (of course crediting your developer in the comments).