The Guardian published this cool but disturbing little Flash gizmo a while back, that reveals which agencies hold what information on you - and who they share it with.
Clicking around reveals a few mildly interesting factoids: the Child Support Agency, for example, links to exactly as many other agencies (in the private, as well as the public sector) as the Police National computer - and both are linked to far more than the Passport office.
And who owns the most comprehensive name and address database in Europe?
Television Licensing Tracking! And they're not even public sector...