Sadly the issue returned.
I've trashed Mojave again and reinstalled office and a few minor other applications. Without DisplayLink installed and at 17:39:26 each day the WindowServer process goes high CPU to approximately 40% and remains there until 18:10.
I suspect that with Displaylink Drivers installed the problem is more pronounced because of the fact that I think the Drivers are trying to stay in sync with the DisplayServer. This means that the DisplayLink Drivers hit about 140-160% CPU while WindowServer hits 40%. Heats the MAC like no tomorrow.
I've been trying to get to the bottom of what is happening but appears that something sets the kernel off and it says that it is flipping the Display. The logs for this vary between occurring every 20ms to far less than.that. I've got a message over at Apple Communities but absolutely no responses at all. Its getting detailed. Last resort is back to High Sierra and surrender on Mojave to see if the problem goes away. This does not seem like a Displaylink issue but it is the obvious visible impact.
https://discussions.apple.com/thread/250645982