I use spirit vinegar that comes in a medium sized bottle for about 70p. This is clear and slightly stronger than malt vinegar. It is normally used for pickling eggs etc. Either wipe or spray on the surface, great for sinks etc. Or put some in a small container and leave the shower in it to soak for a while (you will see it fizzing while there is still limescale). Rinse with clean water and you should be as good as new. A few drops to the water you clean your windows will also help prevent water marks.
Keith Mortimer, June 2004