hi there
you can be a little more ingenious, cut a whole in the side of the water butt at high level, and fix a standard ball valve, with a bit of copper on the other end, therefor you can connect upto to the client tap outlet and leave it running, then you can either syphon out to the pressure washer, or cut an outlet at low level, and insert the fitting and a piece of copper and away you go.
regards
martin