Thanks for the various replies, very interesting. It seems to me that there is no definitive solution. An upturned hull to sit on and call for help would be pretty serious but to see the boat blow away is my biggest fear.
I know that when the kill cord is pulled the whole rig stops pretty fast ( done it by mistake ) but in a following sea in a blow I don't fancy my chances of swimming to catch up to it, even 10 meters with all my clobber on and I'm a half decent swimmer...
A heavy beam landing in a confused sea is the one likely to get me..
I also don't fancy a short harness as, like in a car sometimes, it's best to be thrown free, (although it clearly can stop you going over the side in the first place ) . As long as it was just the right length, sort enough that if I lost my balance the kill cord was activated and I'm not flailing around the transom/ engine ie somewhere just behind the drivers seat. Otherwise I would consider a coiled length of thin rope , which would be just longer than the "stopping distance at say 30 knts " , held with a very thin Velcro strap so it would un-coil under pressure....