Its generally a circular plastic fitting which unscrews in an anticlockwise direction. Unscrew it and then give it a sharp tug towards you. If its stiff, grip it with a pair of pliers. Be prepared for some spillage. To refit, push in firmly till the threaded part engages and then tighten clockwise.
DIY Service manual - www.2ndwave.co.uk/manual.html
dannyboy, April 2009