To ask the Chancellor of the Exchequer (1) what estimate he has made of the cost to the public purse per household of the total increase in the tax threshold over the last three budgets for (a) couples with at least one child with gross earnings between £7,000 and £20,000 per annum and (b) single people with gross earnings between £30,000 and £40,000 per annum, taking into account the reduction in tax paid and the savings in housing benefit and council tax expenditure caused by the increased individuals' net income arising from the increase in the tax threshold;