Should have provided more information, sorry.
So to get this working, I had to do it directly on the camera, there is no option for me on the NVR. I use the NVR for basic motion detection.
Then on the camera directly, I set up VCA > Smart Event > 2 Set Application > Smart Event > (choose...
I contacted Hikvision support and managed to find the setting (may be purely my fault). This isn’t available on the standard motion detection but can be used in with any of the smart events/tracking like intrusion detection, line crossing etc. Got it all working in the end
How annoying when they still advertise it as a feature. Feels like they are mis-selling to people. I tried a firmware downgrade but that also seems blocked