So, placing the helicopter runway over the access road wasn't a good idea - there wasn't enough room to manoeuvre. The better option was to place it at right angles over the grass and make it invisible (1ft width). With that done, along with the custom scenery, the visiting Huey is now working fine in both FSX and P3D.
This particular MAF (Missile Alert Facility) is Bravo-01, about 35 miles south east of Minot AFB:
Bravo-01 is one of 15 similar launch sites, spread around Minot. It's too big a job to make all 15, but I am intending to add Kilo-01 to the west and one more to the north. With all the models made that's quite doable in a short time span. That should allow me to move on and get the whole Minot package assembled and uploaded shortly.
John