In reply to B.Roberts: hi there,
The actual Shunt is the worst device for self-belay top roping.
Use a Petzl Basic with a simple chest harness (even a bungee will do just to keep it upright) or Petzl Croll/Ascension will do the same trick.
Make sure you choose a device that uses the same principle as the above and you won't have to use your hands to drag the device up the rope. Always a good idea to carry an abseiling device with you, in case you have to retreat, and coil up the rope and hang it on the bottom of your line for a little added weight to keep the running of your Basic smooth.
Unfortunately people use shunt because they don't know better. It's one of the great climbing miths.
If you need any more help email me on attilaforbauer@gmail.com