In reply to pottsworth:
I got one very cheap from nevisport in fort william a few years back, dont bother they're shit. If you load the rope in the groove, it jams annoyingly tight and its hard to lower or do anything else really. Also, the recomended lowering procedure is to run the rpoe back over the top of the device, into the notch visible above the V groove. which doesnt work, the rope just slips off into the groove again.
In conclusion, they're crap.