One off freelance work

I am a clerk for a small parish council (employed). A neighbouring parish council has asked me if I can cover their clerk for a meeting. If I do this I will need to invoice them my time. This is a one off. I need to know if I need to declare it to HMRC. Will be a few hours work under £100 work. Knowing will obviously help me know how much to invoice them. Would this work come under the trading allowance? Never done anything like this before so not a clue! Thank you.

Hi,
If this work is carried out as self employed and your gross self employment profit for the tax year is below £1000, then the fee is not taxable and you do not need to report it to HMRC.
Thank you.