Sorry, I should have provided more details:
Latest QGC from the Play Store, and I also tried Daily Build.
I'm using the ESP8266 WiFi module, built and flashed from sources here: https://github.com/dogmaphobic/mavesp8266
I've used the ESP8266 for telemetry to QGC / PX4 and controlled and flown successfully before, but using a much older version of QGC. The one where you could dial up altitude on a slider on the RHS.
Yesterday, as a new development, I got this error message from QGC on Android 7.0 on a Samsung 9" tablet: "Unable to takeoff, MIS_TAKEOFF_ALT parameter missing", this using the latest stable version, and not daily build. Perhaps the telemetry was compromised on the day?
Previously I could arm and take-off, but the Fly/Action button remained greyed out.
Without wishing to complicate matters, I'm also separately testing WifiBroadcast, a medium-range best-effort two-way WiFi UDP telemetry/video approach using Raspberry Pis. My attempts to control the remote craft produce the same results with the PX4/QGC setup as with the ESP8266.
Essentially what I need to know is why the Fly/Action button remain greyed out, without delving into the code.
Thank you for your kind assistance.