OP, every tax-payer is obliged to file a tax return in October for the preceding tax year (Jan to Dec). The fact that your employer is calculating your tax each week / month does not mean you are paying the correct tax, as there may be certain allowances / credits you are entitled to that you are not receiving e.g. rent, union dues, fixed expenses, etc.
Your employer is only aware of the credits as notified by the Tax Office - if you don't claim, your will overpay tax. On the other side, if you have erroneously been granted credits you are not entitiled to (as happened to me after my divorce) you may under-pay tax and the longer this continues the bigger the problem gets.
If you are still unsure talk to the tax office (I have found them very helpful) or a tax advisor (costs are tax deductible once they are registered).