Private Bills are usually promoted by organisations for example local authorities or private companies to give themselves powers beyond, or in conflict with, the general law. Private Bills only change the law as it applies to specific individuals or organisations rather than the general public. Groups or individuals potentially affected by these changes can petition Parliament against the proposed Bill and present their objections to committees of MPs and Lords.
Further information on this subject can be found from the following links.
Private Bills before Parliament
About Parliament: Private Bills