Heat vinager in the microwave until almost boiling in a glass bowl. Put your shower head in and soak over night! Hot vinager is also perfect in a spray bottle (don't heat in the spray bottle as it will melt!) to melt away soap scum and hard water stains on shower walls, glass shower doors, sinks, counter tops, etc. The smell is strong, but it doesn't last long.
Aldi do one in a plastic bottle which descales kettles and coffee machines - I find vinegar takes ages and the descaler takes seconds. I'm sure Sainsbury's and Tesco and the like do one too.