[mungee]

Ooooops! (a big one!).

I have just checked the 'difficulty' settings on the mission concerned and some how or other most of the settings have been changed. After adjusting these, I can now take off from the runway again.

Gee ... I feel silly! It must have been a recently-installed mod that changed these settings ... that's all I can think of!

Sorry to waste your time (those who responded).