In reply to Simon Pollard:
If you don't want to carry extra kit, then a few extra krabs on a bight is the best idea as it will definitely slow you down. Otherwise you could use a GriGri or a Shunt, which'd give you total control to the extent that you could have a complete hands off stop. While effective, a GriGri doesn't give a particularly smooth descent and concerns have been voiced that it doesn't always lock up. A Shunt's 100% better in this respect, always locks up, even on wet ropes, skinny ropes etc.
Lots of people don't like them - too complicated, extra kit/ extra weight to carry blah blah blah... Much the same could actually be said of using a Pirana.
I use a Shunt all the time when abseiling and have done for years. The extra weight is totally insignificant. I do a fair amount of new routeing on sea cliffs and having total control, being able to easily stop and let go to clean a pitch on abseil or generally check out the route is a massive advantage. It's also hugely increases the safety margin, which isn't an insignificant reason for using one either and, unlike a Pirana, you can also prusik with just a single Shunt.
As a caver I've also used a Petzl Stop for years and I see that other posters have recommended it. Using one would be total overkill IMO for what you need and I would never contemplate using one when climbing. Unlike a Shunt, it's big and weighs a fair bit too.
HTH
Dave