I said I'd report back about my first experience of the overnight sleeper - well, we did it, and I'm glad we did, although all those who predicted not much sleep were right!
It was fun going to Euston and boarding the train when everyone else was running to catch the very last train home - yes, the berth was very small but well-equipped; we weren't right next door to the loos so it wasn't too noisy (though to go for a wee in the night I had to get out of the upper bunk, get dressed and stagger down the corridor 
).
We went for a drink in the lounge car before going to bed and there were a few people there who were clearly seasoned regulars. In hindsight I think if I did it again, I might go straight to bed and try to get to sleep before the train left, as I'd probably have more chance of staying asleep. As it was, I did have that weird feeling of hardly sleeping a wink, when in fact I probably dropped off OK. Very odd feeling yourself sliding from side to side with the train, though! The coffee and shortbread delivered by the stewardess (if that's the right word to use) was a slightly surreal touch at 6.45 or so, but welcome.
Anyway, it was an experience and I'm happy to have tried it - I do quite fancy the longer journey through to Fort William so you can actually have dinner on the train.