The tax gap is the difference between the tax in theory that should be collected by HMRC and what actually is collected. At its formation in 2005 HMRC committed to reduce this gap.
http://nds.coi.gov.uk//content/detail.aspx?NewsAreaId=2&ReleaseID=421295&SubjectId=32
seen at 09:18, 23 September in News from Department of Constitutional Affairs - national.