I've had a look at what other scenery files are affected when the add-on option is chosen, in my case with a flight from EGTF (Fairoaks) to EGHA (Compton Abbas) - both ORBX add-on sceneries. As designed, both EGTF and EGHA are enabled, however EGTF displays elevation issues. Among the many disabled files is one FTX_EU_ENG_05_Scenery. I found that when this is enabled the elevation issues are corrected, so it leads me to believe that this file, and not just the EGTF scenery file needs to be enabled for this particular ORBX scenery to function correctly.
I know nothing about scenery files and how they are used and I'm not trying to be critical of your excellent software. I was just pointing out a small problem I came across when I inadvertently chose the add-on scenery option.
Anyway, thanks again for your support and assistance