How should I categorise/record an "account adjustment" on my current account?

Santander failed to notify me of an interest rate decrease on my student current account from 3% AER to 1% AER, which I noticed a few months after the change. I submitted a complaint to them over the phone, and they called me back a few days later to say that they would compensate me by crediting my account with £25, which was more than the lost interest over the period where I was unaware of the change, so I accepted it.

The £25 credit to my current account appears on my statement as an "account adjustment". My question is: how should I classify this sort of compensatory income in my books and on my self-assessment tax return, e.g.

• interest from savings;
• tax-free gift/goodwill from the bank, similar to bank account rewards;
• regular income that is subject to Income Tax;
• something else?