After chatting with a friend ( mechanic) and some fault finding, the issue was sourced back to a seized waste gate valve and failed waste gate actuator.
This created excess pressure at higher speed/revs and the fact it wasn't being relieved, caused the engine to activate limp mode to stop it blowing itself up essentially.
After replacing the waste gate actuator and freeing the waste gate, I can report the cars performance has returned. All is working as should.
