espace 05 2.2 engine filings in oil, with no warning of oil pressure low

Hi guys, really need your help. I bought a espace 05 2.2 two months ago from a used car dealer. A few days ago I started hearing a knocking noise in engine, didnt sound good so I stopped her and parked her up. I changed oil filter and oil but made no difference so i contacted my used car dealer as it has 3 month warranty and I got her towed to their garage. They done the usual checks and said they found filings in oil and are now voiding my warranty and accusing me of driving without oil. This is untrue. I always checked my oil level and oil warning lights never showed in my dash. Can this be my fault ?? How do I deal with this warranty issue? Thanks in advance