Howard reed nails it.
http://classonline.org.uk/docs/2013_Policy_Paper_-_Richard_Murphy__Howard_Reed_(Social_State_-_Idleness.pdf
Around table 3
The current tax and benefit system produces very high marginal tax rates for low earners who are in receipt of Working Tax Credit (WTC), Housing Benefit (HB) and/or Council Tax Benefit (CTB). Table 3 presents some calculations of marginal rates for people on low incomes in receipt of one or more of these benefits. Anyone on Working Tax Credit with a family gross income of above £6,420 faces a marginal deduction rate (MDR) of 41 pence on each pound of earned income. For people above the national insurance primary threshold (currently £149 per week) and the income tax personal allowance (currently £9,440 per year) this rises to 73%. For people also on in receipt of CTB and HB in addition to WTC, the total marginal deduction rate rises to a staggering 96% - i.e. these people keep only 4 pence of each additional pound they earn. Even for low earners not claiming WTC (for example, most childless people on low earnings) the marginal deduction rate for people on HB and CTB and above the Income Tax personal allowance is 90%
