In reply to oem_uk:
You can use the GriGri unmodified so long as your rope runs smoothly (c10mm clean, dry and not too furry). Just coil and suspend the spare rope for added weight. Careful modifications (the notorious 'Deathmod') can make it feed better and allow you to keep it upright by your chest.
Shunt and Petzl Basic are other popular tools (there are others too, arguably better but less popular). Arguments rage over which will make you die the most. Reality is they all work ok, each has strengths and weaknesses.
Try a forum search for 'Shunt' or 'Rope Solo'... Enjoy (or despair!)
jk