If this is all the income you have there will be no tax or NI due as the amount involved (£5,200 pa) falls below the income tax threshold (£10,600) and the NI threshold (£8,060). (This assumes that the tax and NI on your previous employment was all paid up to date).
Beware of registering as Self-employed. HMRC has rules about who is eligible to do so and who is not and the position you hold may not be eligible. Some guidance here:
https://www.gov.uk/government/collections/employed-or-self-employed