There is a serious issue here, no matter what has happened.
If we take the OP's dd at face value, the money was transferred to her account accidentally. There were six different transactions amounting to £500, so she can't have missed them. She knew it wasn't her money but she kept quiet and spent it anyway. That's theft, and I'd be really worried if my dd did this.
If we accept that the OP's dd probably isn't telling the truth, then someone has given her that money and she is afraid to tell her mother why. There cannot be an "innocent" explanation for this, as the OP would already know about it if it existed. This strongly implies that the OP's dd is being exploited. People don't generally hand over £500 to a teenager for no reason.
The OP's dd may fear telling the truth because she thinks she'll get into trouble, or because the person/people who are exploiting her have threatened her with all sorts of bad things that might happen if she tells. However, she is a child and she needs to be protected. The police can help with this.
If, in the end, it turns out that the transactions were made in error, then the police will be able to help explain to the OP's dd why she shouldn't spend money that doesn't belong to her. She is a minor, so I doubt any further action would be taken. If, on the other hand (as I suspect), it emerges that the dd is a victim of some kind of exploitation, then the police will ensure that appropriate action is taken to safeguard the child.
Calling the police is the right thing to do.