Yesterday I had to call HSBC but they were useless so I'll try again here.
Balance is in positive, available balance is in negative. No pending transactions are showing and the last transaction puts the balance still in positive (current balance) however I can tell there's some pending transaction for £15 which takes the available balance into the negative.
After calling as this was a complete mystery to me it turns out it was Spotify. They explained Spotify hasn't cleared the payment yet.
Whenever i go on overdraft I get a text. I haven't received said text and my balance still shows in positive.
My understanding is that UNTIL Spotify clears the payment I'm not actually overdrawn (as money is in limbo so to speak) and then that will trigger the text and get charged for going into the overdraft. It has happened to me in the past, you have a grace period until said payment is cleared.