Compliance check - delivery work

I did a bit of work for Hermes/Evri for a couple of years. Stopped in 2023. It was just a side hustle as we had a baby on the way and I was only doing maybe 3 or 4 Sundays a month. Probably earned about £250 a month from it on average. I was employed full time Mon-Fri in a salaried role alongside this.

When I joined I was told by the manager that I didn’t need to worry about tax as I would be self employed for the deliveries and wouldn’t be earning more than £12.5K, in fact nowhere near this, and that my other job (not self-employed) wouldn’t affect this. I naively believed this and obviously should’ve looked into this myself at the time, but I took his word for it (this manager was gone within a month of me being there, which should’ve been a warning sign for me).

Fast forward to 2025 and I received a letter from HMRC about undeclared income from a delivery role. I’d stopped the part time work a couple of years before this and, as we were moving house at the time and had a new baby, I forgot all about the letter. (Again, I’m well aware it’s my own stupidity and ignorance that’s got me into this situation).

I’ve just had a letter about a compliance check and asking me to report any earnings from the delivery driver role and there are mention of possible penalties. Given that I probably only went over the £1,000 mark for one financial year, and that would’ve been £2-3K max, how likely is it that this will be followed up and I will be fined/charged etc.?

The bank account I used at the time is now closed, so I’m getting old statements sent so that I can provide the information they’re asking for, but I’m wondering how much HMRC are likely to fine me and whether they really will chase this up to the bitter end.

Any advice much appreciated!