Leo Hickman from the Guardian says...
Biological cleaners that use powerful natural enzymes to clear blockages are also available. Bicarbonate of soda (you may need to buy it from a chemist to get the quantities needed), and vinegar (don't waste your best balsamic; a cheap white vinegar will do) poured down a sink or drain, then flushed with boiling water, can work well too.

But the principle of prevention remains the best option. A weekly tablespoon of bicarbonate of soda followed by a cup full of vinegar down a plughole can help to keep things clear as can a regular flush with boiling water.

www.guardian.co.uk/environment/2006/jan/03/ethicalliving.lifeandhealth
I know this as when I was a student we were too cheap to buy proper stuff
( TGB checking Off Topic is still shit at, Mon 1 Sep 2008, 15:20, Reply)