Each tax year you need to make sure you have paid the right amount of tax. You can claim back for up to four tax years.
The first thing to do is to check your final pay slips to see how much you have earned and how much tax you have paid. Your March pay slip should show "salary to date & tax paid to date". HMRC have a handy calculator for you to use. As this only applies the basic personal allowance you will have to deduct any additional allowances (eg. for tools or professional fees) from your final taxable pay first.
"Paying the Tax" Pieter Brueghel the Younger
If this calculation shows you have paid too much tax you should contact HMRC and ask for a refund. You can ring them on 0300 200 3300 You can write to Pay As You Earn, HM Revenue and Customs, BX9 1AS Make sure you give them your national insurance number.
TIP: If you are ringing HMRC try to ring between 8am & 10am Tuesday to Friday before the lines get busy.