"The self-test and BID has been run, no problems"
Does that mean that the Color Split Screen is NOT appearing when running the standalone BID?

Test the UP2715K on a different computer. Do you see the Color Split Screen?

Disagree. The video card driver and/or the onboard GPU driver are responsible for building the desktop =
two 2560x1440 tiles side by side for 5K3K 5120x2880 60Hz
two 1920x1080 tiles side by side for 4K2K 3840x2160 60Hz

The monitor simply shows what the video card/GPU driver is dictating to it.

Thank you for your reply.

There is NOT a split screen during the BID test.

I don't have access to another computer with a DP but I could possibly have it tested a local computer shop.  The fact that the monitor shows the same fault when connected (a) to the graphics card and (b) the motherboard with the graphics card removed suggests that the graphic card is not at fault.

Thanks, I'll have the monitor checked out elsewhere and report back.

I've had the monitor tested at my local computer service centre and it is showing the same fault using their PC.

Thank you for your assistance.  

I phoned Dell and although the monitored had been registered with them since purchase they were not prepared to exchange it.

I have contacted the supplier and they will be sending out a replacement next week.