If my total income (base salary + bonuses) is 45k, will I pay 20% on the £42,476 and 40% on anything above?

My understanding of this situation is that you will pay tax on anything above your personal allowance, currently £7475 for 2011/12, and your percentage tax will be 'tiered' i.e. only 20% on the first £35000 ( after deduction of your personal allowance ) and 40% on the excess income above £35000.

So in your case if you earn £45000, your taxable income above your personal allowance will be £45000 - £7475 = £37525.

So you will pay 20% on the first £35000 and 40% on the balance of £2525. That's the way I understand it!