>"Drink driving" is a new one on me. ...
That's what it is called on this side of the pond.
>I expect Diesels, like petrol engines, cut the fuel off completely
>on throttle overrun, just as petrol engines do, and inject fuel
>to keep the engine running when in Neutral.
True but irrelevant. When the engine is spinning at higher RPMs the engine resistance sucks a lot of kinetic energy. The amount of fuel needed to replace this kinetic energy is higher than the amount of fuel saved by not using idle.
See page 3 of this web page from VW:
Let it roll.
Use overrun fuel cutoff.
0 gallons used: coast for free
With the right driving techniques, you can really save big. For example: did you know that you can go 60 miles on precisely 0 gallons of gas? You can - by coasting with the car in gear!
The overrun fuel cutoff feature interrupts fuel supply to the engine so that not a single drop of fuel is used. The best time to apply this clever little method is when you’re going downhill or approaching a red light, which is when you can also benefit from the engine’s braking effect.
You’ll also save fuel by disengaging the clutch. This is particularly effective when combined with predictive driving to coast on a long stretch of road.
Coasting downhill is effective, too, but only if the gradient is not too steep.