From the Office of Tax Complication

Even comparatively mild adjustments to tax thresholds can produce unintended consequences, writes Simon Sweetman.
For 2011/12 the 40% tax threshold has been reduced from £37,400 to £35,000. The effective basic rate threshold of course goes up by £1,000 to £7,475. Similar arrangements have been made for NIC with the lower threshold being increased while the higher threshold is reduced (and the rate above the threshold increased to 2%).
Continued...
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overlooked
One thing where the 52% definitely kicks in is if benefits are involved.
Consider:
Salary £40,000 plus car/other BIK of say £4,000 = £1,525 at 40%
But for NI at £40k still paying 12% NI.
Always the case
For someone with a salary within the basic rate band and below the upper NI threshold but with other income - whether benefits, investment income or other - that takes him into higher rate tax, the marginal rate of tax on additional salary has always effectively been higher rate plus ee NI rate - 51% at present.
For someone with a lot of investment income, a marginal rate of 72% will be possible from 6/4/11!!
Office of Tax Complication
If we incorporated employees' NI into personal tax then all these problems would melt away, together with IR35!


Eh?
Surely this article is wrong. Higher rate income tax kicks in at £42,475 (personal allowance of £7,475 and 20% band of £35,000) and 12% NIC as article says stops at £42,484 therefore there is just £9 where the 52% band applies