The FCA’s latest guidelines are that the first £500 of an overdraft should be interest-free for now. Some systems won’t have caught up with this, and you may have accidentally been charged. However, if you went into an overdraft and didn’t have one agreed with the bank beforehand, they can charge you for that. The same goes for if you go over £500 (or, if your agreed limit was say £350 and you went over that) – you’re still going to get charged interest on the amounts above the limit.
Definitely give your bank a call to see if they can refund the charges!