You may be on the right track with your theory that the fuel rail is not holding pressure overnight. But you could also have the same problems with, say, glow-plugs that are not working efficiently (or at all). In that case, the vehicle will be hard to start form cold and could easily display the other symptoms you’re seeing.

Modern turbo-diesels have a pretty complex (and super high-pressure) fuel system, so there’s plenty to go wrong. A check-up at a diesel specialist is probably the best advice. Don’t be tempted to tackle the problem in your driveway, either, as these systems run at sufficient pressure to remove fingers if you get it wrong.

In fact, a diesel engineer once told me that one of the big concerns with these modern diesel engines is making them tamper-proof to protect inquisitive backyard mechanics.