@buchan30 - my niece had this very issue about 5 years ago when she was still at school. It was awful, I don't think folk realise how destructive the relentless nature of online bullying is.
With old style school bullies, at least kids got away from them when they came home. Now it is day and night.
My sister-in-law tried everything, including confiscating my niece's phone in the evenings. None of that worked. For months it was harrowing. In the end, if I remember correctly, she managed to find out the address of one of them and went round to speak to the parents, armed with the messages. It died down after that.