At which rate of tax is any redundancy payment above £30,000 taxed?

In general, redundancy payments are not subject to income tax. However, to the extent that payment exceeds £30,000, they are treated as employment income and chargeable to tax under s.403 of the Income Tax (Earnings and Pensions) Act 2003. The amount above £30,000 is taxed at the employee's marginal rate of tax, ie at the highest rate of tax applicable to the employee in question, as if it were the "top slice" of the employee's income for that tax year.