The Electoral Commission informs me that its 2005 report, ‘Understanding Electoral Registration’, found that the best estimate for non- registration among the eligible household population in England and Wales at 15 October 2000 was between 8 per cent. and 9 per cent. The report is available in the House of Commons Library and on the Commission's website.
In addition, the Commission published in October 2007 the findings of a report by GfK NOP into registration in London: The completeness and accuracy of the Electoral Registers in Greater London—a pilot study, which is available on its website.
The Commission also informs me that it is planning to undertake a review of local Electoral Registers in a small sample of local authority areas. This is due to begin in early 2009.