By sticking the GPS close to the window, I managed to get the GPS to get a lock as the B 777 made it's way along the taxiway. Although the windows in a commercial aircraft are small, there isn't much else to obscure a good view of the sky, so I had a good signal almost the entire way, apart from when the GPS feel onto the floor when I was sleeping.
Annoyingly my GPS (a garmin Vista) records barometric altitude, rather than GPS altitude, so the altitude readings reflect equivilent cabin altitude, not actual altitude, which was most around FL 36 that day.
Note: A GPS is a passive electronic device, and doesn't interfer with aircraft systems, so could be left switched on for the entire flight. The flight crew might not see it take way, so if you do the same, be sensible about using your GPS.