Kleeneze vacuum suction pump, kind of an oversized bike pump with an open ended big rubber seal around the bottom, which you place over the bath/sink/showertray plug hole. Tilt it back a wee bit and draw in water, re-cover plughole and hold firmly. Pump up and down several times till blockage is cleared.
Safe and chemical free. Just cover over the overflow first to avoid splash back.
Scotty
January 2009