Macbook Pro 15in 2018 on High Sierra 10.13.6 won't detect external monitor LG27UK850

This Macbook Pro is new to me, and I'm new to Macbooks. The whole experience is driving me up the wall.

I have an existing LG27UK850 that I can throw any old Windows/Linux desktop or laptop at and it works as expected. Plug in this Macbook Pro, and the monitor highlights that a USB-C cable has been plugged in, and asks if I want to switch to that input. If I do, the monitor reports "no signal detected".

On the Macbook Pro, only the native display is detected.

A friend of mine ( 400 miles away unfortunately ) has the same setup exactly, and his just works.

I have used multiple cables, existing and newly purchased from the Macbook to the monitor. I have tested and confirmed the UCB-C input in the monitor works from other laptops. There is no hub or other device in the loop. USB-C from Macbook direct to UCB-C port on the monitor. I've reset the monitor. Updated the monitor firmware. Played around with the order I plug in. I'm beaten.

I have reset NVRAM and SMC as appears to work for these users on this post ( and lots of others ).

https://discussions.apple.com/thread/8136200

I have built every PC I have owned for the last 20 years, so I know my way around plugging things in and testing them. This Macbook Pro has beaten me.

Before I set fire to it, is there anything else I can try ? This may well be software or an o/s setting that I don't know about as I haven't used macOS before, and after switching to an iPhone a few years ago I thought it make sense to move across for my work laptop as well.

Thanks in advance