Estimates of the numbers not claiming their entitlement to council tax benefit, and the amount of council tax benefit that is not claimed are available in the DWP publication series entitled "Income Related Benefits Estimates of Take-Up in 2005-06". Estimates relate to Great Britain only. Copies of the latest publication, plus past reports, can be found in the Library.
There is no saving, relative to spending plans, from take-up of benefits being lower than 100 per cent. Forecasts of spending on benefits are based on estimates of those who will actually receive the benefit, not those who are entitled. This is in accordance with Government accounting rules that require forecasts to be taut and realistic.