The latest research carried out by the Department for Education indicates that under-registration for free school meals fell from 14% (197,700) in 2012 to 11% (161,400) in 2013. The research is available at: www.gov.uk/government/uploads/system/uploads/attachment_data/file/266339/DFE-RR319.pdf
The January 2015 census data was published in the department’s 2015 Schools, Pupils and their Characteristics statistical first release (June 2015), available at: https://www.gov.uk/government/statistics/schools-pupils-and-their-characteristics-january-2015. The relevant statistics are at Table 3d of the underlying national data tables.
Pupil premium funding is based on whether pupils have been registered for benefits-related free school meals at any point in the last six years and not only those who are currently registered.