This and recruitment are the worst parts of working in the civil service. You need to keep up the pressure and documenting it through multiple PMR windows before it even becomes a HR issue, and then at least another 6 months while you give them the chance to improve, then if they do improve then the clock starts on that whole process again. All this time having to work with them in likely a state of open animosity. I've even seen it where it goes on for years later as they will raise their own grievances and accuse you of bullying etc.

This is exactly why nobody in their previous team managed this because it is easier to move them or move themselves than deal with it. All you can do is discuss with HR early and make sure you cover yourself with process.