Thanks for the picture, it gave me a clue...
All fixed.. but not obvious..
Yes, the vibrations from the router, causes the thread to unwind.. always unwind as it's the weight of the router pulling it down..
I'd moved the red slider back and forward and it didn't change the friction.
The red slider is a locking bar, sliding it back allows you to tighten the brass nut (green arrow)
This has a o ring (Pink line) underneath that squashes the rubber into the thread and applies more friction.
Once tight you can the slide the red bar back across to stop the Nut moving, and finally tighten the cap screws.
PS Does anyone have a manual?