Do you have any photos of what you've got, Richard? Friction steering dampers are certainly not state of the art, but they can work fairly well, and the ability to adjust it is useful.
One thing to watch is to make sure there is no free play between the "fixed" portion of the damper and where it locates on the frame (or sidecar fitting in your case) as the presence of any undamped movement in the straight-ahead position will mean you need to run the damper even tighter to compensate.
If I remember correctly, the adjusting nut (obviously on the bolt which passes through the top and bottom yokes) bears against the fixed plate which is located on the sidecar fitting. In operation, this means the nut has to rotate in relation to the fixed plate which can lead to additional friction which is difficult to control. I fitted a thrust washer assembly here, consisting of a well-greased bronze washer sandwiched between two steel ones. If the bolt is long enough, you might find a needle roller bearing works well:
https://uk.rs-online.com/web/p/roller-bearings/1999805?