You should register as self employed and complete accounts. Any expenses which you incur "wholely and exclusively" in earning this money (i.e. motor expenses) can be deducted as expenses and you will pay tax on the balance. If, for instance the standard rate cut off is 29,400, your current paye salary is 28,000 and you earn 2,000 (after expenses), then you would pay tax on the first 1,400 (29,400-28,000) at 20% and the other 600 (balance) at 42%.
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