Hi tcoatdan Coates,
If you will be self employed with this second source of income and you earn more than £1000.00 in a tax year, you will need to register for Self Assessment, as self employed.
Set up as self-employed (a 'sole trader'): step by step
As self employed, you would need to keep records of your turnover and allowable expenses.
You would be able to claim either £1000 trading income allowance against your gross profit or set your allowable expenses against the gross profit.
The resulting net profit, will be subject to tax and National Insurance, taking into account the National Insurance paid against your employment income.
Self assessment will work out how much this is and will collect it along with the tax payable.
Thank you