These documents provide information on the current methods for producing internal migration estimates:
Internal Migration Methodology (343.1 Kb Pdf)
Internal Migration - Cross-UK Methods Comparison (138.6 Kb Pdf)
Internal Migration Methodology: An Improved Method of Estimating Student Migration - Information Note (October 2012) (44.4 Kb Pdf) - this provides more detailed information on the current methods for estimating student migration, which were first used in the estimates for the year ending June 2012.
Here are questions and answers from earlier years:
These older documents describe the methods used for the years ending June 2002 to June 2011:
Internal Migration Estimates - Methodology (November 2011) (125.5 Kb Pdf)