This is foreign income and meets the criteria for completing a Self Assessment tax return. You can register for a tax return here:
Register for Self Assessment
You would need to convert from euros to sterling, using the official exhange rates, which can be found here:
Exchange rates from HMRC in CSV and XML format
You would complete the self employment supplementary page SA103 and if you paid foreign tax, you would also complete the section 'Foreign tax paid on employment, self-employment and other income' on the foreign supplementary page SA106, as well as claim Foreign Tax Credit Relief.