When an individual identifies a reason for filing a Self Assessment tax return, they need to tell HMRC. You can do this either by completing an online HMRC form or by downloading and posting a SA1 form to HMRC.
It takes around 6 weeks for HMRC to process and once done, they will post a letter confirming registration and provide you with a Unique Taxpayer Reference (UTR) number. You need this to file your tax return.
As soon as you receive your UTR number, please email a copy to us at firstname.lastname@example.org