Thanks for the suggestion Umberto but, honestly, I'm as sure as I can be that there are no other Afcads involved.
I usually check for duplicate afcads by using Scenery Configuration Editor to expand local paths and put the resulting scenery.cfg from P3Dv4 into FSX and use AFX to check which afcads are active.
I tried a similar trick using the simple-airport-scanner (which only officially seems to work up to P3Dv3) with the same result.
As far as I can tell I'm seeing the default markings with the FSDT markings at a higher elevation.
I've no doubt it's something on my system - just running out of ideas what it might be!
Again, thanks for the help.
I'm away for the rest of the week then will try and find time to look at this again.
Thanks for your wonderful scenery.
All the best,
John