Also much better for the environment. Having said that, the Dundee - London train is a diesel, because no-one has electrified those lines.
i really like the idea of a sleeper, and have used them a few times: Paddington - Penzance, London - Inverness - London, Aberdeen - London, Fort William - London, London - Glasgow, London - Edinburgh, Cologne - Vienna. But it's the practicalities that put me off. The terribly cramped compartments (I think it's a bonkers idea to put a shower in a sleeper, which we had on last year's Vienna jaunt. Takes up ridiculous amounts of space, and last year when I got to Edinburgh I just had a shower at the station), the fact that I don't sleep very well, and also that I like to admire the scenery which you can't really do at night. When we returned from Austria last year, the Salzburg - Frankfurt train went through some wonderful countryside. That's an important part of the holiday from my point of view.