[stepheno]

Firstly buy local - which may, likely as not be Jessops, or from somewhere that it is easy to return to. Secondly try the lens in the shop - on your own camera, and if necessary ask to view the shots on there computer or take a laptop with you. Don't take the imoages home to view since next time you go to the shop it might not be the same lens.
If you are happy with the results then buy the lens. You then have two safeguards - firstly the retailer will have a "return for a full-refund" clause, usually something like 21 or 28 days. Secondly, if out of the refund period, you have Canon's warranty so in the event of a faulty copy you return to Canon UK who will arrange for an accredited Camera Repair Shop to fix or Canon will replace. I can't think of anything better than that. Obviously the nearer the retailer is the easier it is so if you buy from Hong Kong you will get the best price but also the hassle of returnigna poor copy. HTH