In reply to G3CWI:
I too have found using a GP vertical at elevation be a SOTA summit or a hill with a good to view to the horizon gives results that are just not possible from home.
I get your point about the drop off on Shining Tor, Something I have never thought of before, I have had several good easy contacts into the States and Canada from Gun which as you know is almost flat facing West.
What I have found with GP verticals is getting the ground planes off the ground, With the first one I made for 20m when used on the shorter 6m SOTA pole the radials were fairly close to the ground, So far the 20m vertical has given the best dx!
What angles are the radials on your verticals? How many radials for the GP do you use? The angle is something I found to make little if any difference in performance although I could be wrong. I hear two radials will give approx 90% efficiency with three or more being best, For this reason I settled on three radials which as you know also serve as guys for the pole.
When you was on 17m yesterday I could not hear any other stations apart from yourself, Setting up a GP vertical on a summit compared to being at my home QTH is like comparing night to day!