The OBS softkey is enabled when a flight plan is active.
Sounds so simple when you say it! Perhaps that piece of advice is in the documents somewhere, but it certainly passed me by - many, many thanks for the insight.
The book you mention doesn't seem to be easily available in UK so I'll wait until I am next in the US. Meanwhile, I'm continuing to find stacks of stuff on YouTube, though it's taking a while to carry out a wheat/chaff sort. I wonder if the fact that there is so much there supports my impression that Garmin's own extensive documentation is not clear enough? I do feel a bit mean about criticising it, because it is certainly detailed and - mostly - comprehensive; perhaps it's my own fault for expecting it to be something it wasn't designed to be.
But my main message is - thanks again, Soay!