With regards to PAYE and tax returns, if you are paying 40% tax (over about £37-40k) you will also need to fill out a tax return, or will do soon. It is the stated aim of the Inland Revenue to get ALL top rate tax payers to fill out a tax return, once you have filled one out for one year (say you registered a capital gain from selling some shares) you will have to fill one out every year.
The odds of getting a refund of overpaid tax is VERY low, as all allowances are generally built into your tax code, my experiance is that every tax year I owe the IR a few quid, which if it under x amount is just added to your tax coding and paid throughtout the next year.
Tax percentage on £30k works out at around 30%, depends on whether you have kids (working family tax credit). We have income tax (0% rate for first £4895, then 10% of the next £2090, then 22% for the next £30310) for a gross of £30k this would leave a liability of £5272.30. Then you have National Insurance, according to our Dear Leader this is not Income Tax, but merely a tax on income! This is paid at 11% on income between £82 and £630 per week, so on your weekly salary of £576.92 you will pay £54.44 per week x 52 = £2830.96 per year.
So your final tax bill will be £8103.26 on a gross of £30000 or 27%
If you have one child and your wife does not work, you'll get £535 a year as a tax credit (paid into your bank account directly)