my DT 125 (LC1) runs okay but the slow run mixture adjustment screw does not respond at all. The carb has standard settings (jets) fitted and the exhaust system is standard fit to ?

Are you talking about the idle speed screw? (Big long one sticking out of the carb) Or the air screw (little slotted brass screw on the side of the carb)?

The slow run mixture screw (air screw). The Mukini carb setting is about 1.5 turns out but either fully open or closed does not affect the idle speed much

sometimes Alan I have seen a small jet type orifice right in the mouth of the carb where the air is sucked in for the mixture screw regulation,,,not sure if your LC has this but they can get a bit blocked with debris from the airbox and need poking clear and blowing through with compressed air. You can see it here right at the bottom of the intake..

 

When the Airscrew is screwed fully home,  the engine should stumble and conk out,  so its either needing cleaned [ as Airhead says]   ,OR. the pilot jet is too small, 

Okay cheers. Although the carb was ultrasonically cleaned I must admit I never checked the pilot jet size ?

One other cab question is what wear checks if any required for the Choke Plunger unit. Do these ever wear out ?

I"v never  seen a worn choke plunger,  they get little use in there lifetime, , slide and needle wear is more common,

Ultrasonic cleaning is not perfect either, compressed air and some fine copper wire (wont hurt the brass or ally) is better.

Also is there an O ring fitted to the threaded part on these units ?