Hi Amanda,
1. If the account is in joint names then both account holders need to write in and request for the account to be closed. The bank/building society needs the consent of both account holders.
The other point is that if it is appearing on your credit report, then any adverse credit on the account will impact your credit score.
2. The link below is to help find dormant accounts and should help....
http://www.bba.org.uk/bba/jsp/polopoly.jsp?d=1 44
It helped me find a sving account of mine some 20 years later with a fortune of ...wait for it...drum roll....3pence!!!
Good luck and hope this post will be of help.
Raj