Get Motability involved; they "Own" the car, in a way. Also see this similar discussion

http://www.honestjohn.co.uk/forum/post/index.htm?t=93410

Carole4x4

driving a diesel will be a very different experince to a petrol, I must also add that most diesels new are very tight and very thirsty. My Own diesel a L/R when new was returning 23ish and has improved to 36 ish after 10,000 miles and my Wifes 1.6TDCi Fiesta would not better 40 tops untill about 9000 miles and now its 60+ and very rarley drops below 54 with constant town driving.

As for the tourque it may feel sluggish after a petrol but try a different approach to driving it. Reving it will achieve nothing other than drinking fuel. Our fiesta only needs driving between 1200-2250 rpm and thats plenty and we get the best economey. If you driving it like a petrol and rev it to 3000+ between gear changes the MPG will plummet.

You will also need to deal with mobility and have you driven another Megane the same spec to make sure yours isnt faulty ? dont take the dealers words that its fine.

Recently had a customer who had a new Fiesta 1.6tdci that was really flat, went to the dealer over 10 times and they said nothing wrong. He went back to the dealer and hired a 1.6TDCi and went like a rocket. He went back to the salesmanager and asked him to drive both cars ! The manager then agreed something was wring ! Ford then became involved and there was a problem that required an engineer from germany to come over and problem was fixed So never take what the dealer says as gospel !

Curious by the driving charecteristics having " changed so dramatically" I went & test drove a low mileage 1.5DCi & a 1.5dci eco Dynamic to see for myself if there was this huge difference that would make one undriveable !

Well figures for performance are virtually the same & the Tourque output quoted was 240nm @1750 rpm for both engines .

Across the range of 1.5dci engines there are 80-110BHP versions but with the exception of the 90 & 100BHP models the tourque remains the same at 240NM

I found very little difference between the two except the eco model was brandnew & tight & could appreciate once it had loosened up would performe as one would expect.

That leads me back to my original question, is "Carloes4x4" Renault running correctley ! are the dealer just blaming it on the "new eco engine" and hiding behind a fault that needs exploring just as in the scenario of the Fiesta that I quoted !

or is it a DPF issue, & that the vehicle cant be driven enough to regenerate when required ! may be some folk dont like the idea of having to give it a blast down a dual carrigeway.

I think the reason as to why it cant be driven needs elaborating on as oldtoffee quite rightly says.

Sounds like you enjoy the challenge of maxxing the potential fuel economy of your cars (and driving the proverbials off them too!) Leaving aside stress on clutch and dual mass flywheels l'm not sure a highly geared diesel is going to respond with best mpg to keeping the revs ultra low as you descibe. Ford with their Focus Econetic deliberately chose a 5 speed manual box as in real world driving it gave better mpg as revs were that bit higher across the range.

Be interested to hear how you get on with the comparison. Just out of interest, what engine do you have in your Discovery and what mpg do you get?