If you have reason to believe that they have not supplied you with all data held about you, both computerised and manual data, in response to a SARN; Then send a letter to the Information Commissioner requesting an assessment. You can download the assessment request form from the IC site, but a letter will do.(posted 8106 days ago)You can telephone the IC on 01625-545700 and they will advise you on such issues.
You need to prove to the IC that there is missing data. Send them copies of letters etc. to prove your statements.
Good Luck.