Mike K wrote:
As it’s not a regular income, could I make one off payments to HMRC at the end of each year once I know how much I’ve earned?
You need to pay taxes on any self-employed income over £1,000, in addition to the taxes that you pay at your regular job. Information on registering your business is here: https://www.gov.uk/set-up-sole-trader. If you've never done taxes as a small business before, then it would be best to hire an accountant for at least the first couple of years to ensure that you're filling out the information correctly and claiming all eligible expenses.