Thanks, I had to set the mode of the binding to diff-mode to stop it affecting the normal binding I have.
I discovered something odd - if I select two revisions and right click - "diff selected revisions" - I get a different diff dialog than if I select two revisions and click the toolbar button "diff selected revisions". The "backup history version" means I have to click the mouse in the "source window" to get ctrl-down to work whereas the diffzilla pro window already has focus in the source and ctrl down works immediately. The diffzilla pro window takes longer to come up though.