If coasting in gear, the engine is still running but burning minimal fuel. If there was no fuel being used the car would die, just like it does when you run out of fuel.

My tips for eco driving, make sure tyre pressures are correct, leave plenty of space between you and car in front, anticipate and roll off throttle early, or walk  

If coasting in gear, the engine is still running but burning minimal fuel. If there was no fuel being used the car would die, just like it does when you run out of fuel.

How would it? The fuel is only needed to explode in the pistons in order to turn the crankshaft at idle RPM. If the gearbox is doing that for you, then no fuel is needed until you have to put the clutch in to come to a stop.

low revs means little work.

In gear for the same decline means higher revs and more energy lost due to heat, noise etc

Autumn doesn't help. More people back at work, more stop start, denser cooler air so more drag. Does the anti waxing chemicals in Winter diesel affect fuel economy?

In gear for the same decline, as opposed to neutral? Yes, of course it means higher revs, but more energy lost? I assume you're referring to the fact you're performing engine braking and therefore you're losing your kinetic energy faster than you would do normally. However, you're not directly paying for that kinetic energy, it's a byproduct of the energy you've put in on the approach to the hill. Coast down in neutral and you're pretty much throwing away all the kinetic energy you've built up, and then using the fuel you pay for in order to keep your engine ticking over.

The higher revs are nothing to worry about, oil etc is circulating as normal, so they don't affect anything at all IMO. I've read that winter diesel can affect economy, but tbh using Fuelly I didn't really notice a difference last winter (either in the summer-winter switch, or back again). Maybe it depends on what formula different fuel vendors use for their winter diesel, I'm imagining that there are a few ways to achieve the same goal.

In a nutshell all you need to think about is NOT labouring the car in all situations and its been well maintained, mine was sitting at 63MPG today with ease

Yeh keep the car in gear when going down hill..uses more full when the cars at idle engine speed than when its in gear with no throttle..

Also my hgv trainer advised me that basically dont let the vehicle stop..keep it moving..so anticipating ahead..slowing down early..as a lot of fuel is used to get the car moving!

If slowing for a junction or in traffic on the m'way (M25... :( ) , it's easy to see trundling in neutral uses a small amount of fuel, whilst using the engine to slow in gear, the instantaneous MPG display blanks to dashs "---". As it can read up to 180 MPG, I'll take that as evidence that slowing in gear is better than in neutral. EGR also may have something to do on the overrun. I'm a little unsure as diesels aren't air limited.

What I can believe is that descending in neutral adds speed whilst descending in gear is likely to slow you. Hence if you've got to climb back up, it could be more efficient. Though allowing the car to develop a slight "head of steam" whilst in gear is likely to have much the same effect IMO and also IMO be a safer way to do it.

Get a remap for economy, change air filter every year, drive like a granny and bore all your passengers to death.