Latitude 5300 USB C Power Issue

Well I've got a pretty strange issue...
Picked up a Latitude 5300 recently, it was sent with a USB C power supply, however, the laptop won't charge with it. When I plug it in it says (after a few seconds) your laptop is not charging, make sure you've plugged it in correctly (or something to that effect). So it seems to recognise that it's receiving power.
I have confirmed, with another laptop, that the USB C PSU is perfectly fine.
The laptop charges fine with a normal PSU in the standard Dell power socket.
I have confirmed that the laptop has power delivery over USB C.
If I connect a Dell USB C Dock (WD15 is what I have, though also tried a WD19) it is recognised and functional, as long as power is connected to the dock.

Here's the even more interesting part - If I connect a standard Dell PSU, then connect the USB C PSU, then remove the standard Dell PSU, the laptop will happily continue to charge over the USB C PSU!
I have confirmed this also works with my WD15 dock.

So it seems the issue is that it's unable to initialise the charge over USB C, but once it's charging, it's happy to receive power from anywhere.
As soon as I unplug the USB C and replug it, it goes back to the previous message.

So I have 2 questions:
1) Does anyone have any idea what might be causing this and how to fix it?
2) Failing that, does anyone know how I could trick the laptop into believing that it's charging (with some sort of script perhaps) so I could run something and force USB C to start accepting charge?