I think I've gone some way to solving my problem. It was a case of going back to basics and systemically testing everything.
The upshot, after lots of trials, indicates that the Massoth PC Module doesn't work that well with it's firmware update to version 3.2. I've reverted back to firmware version 3.1 and can now consistently read the CV values from LGB 25522 (V137 diesel) using the latest DCC Programming software (vers 1.4.16.1).
Reading the CV's from the DR 99 6001 steam locomotive derivatives were improved but still could be temperamental. Using an improved power supply resolved these problems and it now appears that the Massoth USB PC Module operates near faultlessly with all the locomotives.
I don't know why it should be a problem with 3.2 firmware. From past experience it sometimes is related to a non sequential application of updates. The Massoth PC module was bought new about 2 weeks ago but surprisingly it arrived with 2012 software.
PS Keith thanks for your reply. I'm in the same position as you with MTS III and Massoth navigator, hence needed an easier way to read the CVs. I'm getting too old to squint at little handsets to adjust the values

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