How to copy an old revision from another subversion branch to the head revision on trunk?

I have a repository where I have branched and merged back in some changes in to trunk. However I need to now revert trunk back to a revision on my branch.

I imagine the solution is to use svn merge however I have not succesfully been able to use this accross branches.

Any suggestions would be very useful.

Move the old trunk away

svn move http:///.../trunk  http://..../branches/abandoned_trunk

And then copy the old revision to become the new trunk

svn copy http:///.../branches/the_branch@1234 http://.../trunk