Frequently asked questions Can I complete my return after the January 31st deadline? NO! If your tax return is not submitted by 31st January you will get an automatic penalty of £100. A further £100 will charged if your return is still outstanding after six months. Plus interest on unpaid tax attracts a 5% penalty. Can I estimate my income if I don't have all the information in my tax return? How will I know if my tax affairs are being investigated? I think I paid too much tax, how do I get it back? © Taxes. All rights reserved.