Best not to use a chemical at all, under the sink is a U bend, the pipes can normally be seperated either side of it, with an empty bucket under it, release the pipework an clean it out.
If you really must use chemicals i've used a product called Drain Away
that is really good http://www.amazon.co.uk/Drain-Unblocker-Sachet-Cleaner-Showers/dp/B009S5WBXW/ref=sr_1_3?s=kitchen&ie=UTF8&qid=1361999884&sr=1-3
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