Unfortunately after getting no contacts from the three J-pole antennas on 15, 17 or 20m, I decided not to test the QRP GUYS antenna from the summit as iot was always going to be a compromise antenna and it looked like a storm was heading my way, so to qualify the summit, I put up my old SOTABeams linked dipole and worked a total of 36 stations on 40m.
So a test of this antenna from a summit (or portable somewhere), is still on the “to Do” list. Most likely next Friday on Laber.
73 Ed.
UPDATE APRIL 6th.
After re-building the QRP-GUYS tri-bander as the standard 20/30/40m model rather than my 20/40/60m model I found THE SAME PROBLEM.
Changing the length of the coax feed changes the resonance of the antenna on all three bands! even adding a common mode choke into the coax feed made no noticable difference.
So this appears to be a fault of the design, not my changes to the design.
As a solution, I have cut a length of coax with a BNC plug on it and soldered that coax directly to the board and then re-adjusted the antenna to be resonant across 20, 30 and 40m. Lets hope things stay about the same.
Martin’s design seems to be a lot better than the QRP-GUYS one. 
73 Ed.