The most often quoted stats (such as the 'its safer than driving' line) are referring to commercial flying, and there are plenty of stats regarding that.
Most of the stats I've seen regarding GA and light a/c flying rate in a similar risk catagory to riding a motor bike. I'm pretty sure I saw some stuff produced by the DTI which covered these figures. Also the insurance companies do their own risk assesments and incident severity analysis based upon claims and hours/cycles flown - so if you ask them nicely they might give you some of thier in-house stats, although it could put you off flying for life.
However, when a pax ask's me if it's safe I admit I try to calm them down (by bending the truth slightly) and saying we were more likely to have an accident on the way here than we are whilst up there.