I forget to say, if I replace the CSV-1585 with a Dell DA300 , then it works perfectly at 4k 60hz

You must create a custom resolution with reduced timings. This combination of devices uses “safe” video timings, which limits the available bandwidth. You can change this in the Nvidia, AMD, or Intel control panel. When creating a custom resolution, the refresh rate should be 4K60Hz. The only setting to check is the “timing”: CVT RBv2 or CVT Reduced Blanking.

Right, well this is used in a public meeting room, so random people should just connect it and have it working. You can probably see my predicament…

In my view the problem is with the device since I’ve tried several more laptops and tv’s now and it used to work just fine with the old dock, so if the device cannot be fixed I’ll be returning the unit. 

Thanks for the efforts!

One last thing – could you check that HDR is disabled from the laptop?  it could be that taking Enabling HDR since display is supporting it that the bandwidth is on the edge of 8bits. toggle off HDR function to see if 4K60Hz can be forced?