I don't know much about diesels but,
I used to own an audi coupe quattro. Sometimes it would start and run for a second , then stall.
This went on for months. It wouldn't restart after either. Then for no reason it would start.
My brother in law had the same problem on a fiesta. It turned out that the fuel pump fuse contacts were dirty. Pulled the fuse out, cleaned it, put it back. Perfect from then on.
my point here, could it be something similar. Perhaps a dodgy contact on the fuel pump.
Also, previously to this audi I had a ford KA. This too started stalling when I slowed down from high speeds. Turned out to be some sort of air sensor. (it was 10 years ago and I can't remember exactlky what it was called)
hope this helps
If you think you're a hero, check you have a guitar