The losses can be dealt with in several ways as you can reasd in HS227 here.
If you wish to claim relief against your other income for the tax year in which the loss was incurred, you need to put the loss in box 33 of the short self-employment pages or box 78 of the full self-employemnt pages. That should result in a tax repayment assuming the correct tax has been deducted from your salary. If you carry the losses forward, you can only offset them against profits from the same business.
I hope this helps but let me know if you have any further questions.