Matt, you might be thinking of a ball joint splitter there.
I had to do CV boots for MOT recently, I found the best way to remove the joint, was to remove the driveshaft from the car and set it up in a vice to work on. It will probably require quite some force to remove the joint, a 'tap' as it says in Haynes just won't do it!
Be prepared for the balls falling out too, work somehwere well lit and clean so you can find them if they go walkies.
Also, a good time to refurbish the joint itself, give it a good clean up and reinstall with a new boot kit, new grease, circlip and hubnut/bolt.