Hi Nick and John,
While struggling with this problem I've uninstalled EGNT and had Orbx Central reinstall it several times, all to no avail. This morning I remembered a collection of old UK2000 airports which still work in P3D5. Finding one of them is EGNT, I installed it. It does look good!
For some reason which escapes me (loyalty to Orbx perhaps?) I uninstalled it, and once again went to Orbx Central. This time, for no other reason except despair, I had Orbx Central run as administrator. OBC then installed EGNT, found fault with two files as always, and verified files ok.
I opened P3D5, loaded a flight at EGNT, and lo and behold, I have my old and favourite airport back!
With Thanks and Best Regards, Jim