If they are pursuing you for this shortfall have they sent you a letter chasing?(posted 7569 days ago)If they have simply send them a copy of the latest letter along with a cover letter making reference the SARN received(careful - acknowledgement).
If they have never sent you a letter then wait until they evenutally do and point the cml policy out to them, Nationwide are a member.
See other postings on this site regarding acknowledgement and also CML shortfall policy
Make sure you send any letter recorded delivery and get proof of post.