I don't know the answer to the card filling question. I have returned products to Canon UK for warranty work and never filled out a warranty card. They have merely requested proof of date of purchase.
I believe you got your lens from outside the UK. I know of people who have tried to return them to Sigma UK having imported them and have been told that Sigma will honour the guarantee but ONLY if you have proof that you paid VAT when you imported the lens. (that may include proof of import duty also, I can't remember, but if you were billed for VAT then import duty would have been added as well)
The best way to get a direct answer to your questions is to ring Sigma UK. They are in Welwyn Garden City, Hertfordshire and their number is 01707 329999.
Their website is
http://www.sigma-imaging-uk.com