Police checks and clearance forms

Questions:

• How can I find out what information the police hold on their systems about me? I want to check it is correct.
• I had a relative in the police force and he/she is now deceased, how do I get information about their time in the service?
• I need a police check for immigration / emigration purposes, what do I have to do?
• I received a Fixed Penalty Notice (FPN) and paid the fine. I am currently filling out an application form for a DBS check. The form asks if I have any previous convictions, cautions or reprimands. Do I need to mention my FPN?
• What does a spent conviction/caution mean?