So, unusually for these forums, I'm going to tell you all about the symptoms, the things I tried AND the final solution to this problem.

It's a 2005, with about 130,000 miles on it. It gets no servicing other than regular oil and filter changes (oh, and a cambelt). I've owned it for about 60,000 miles.

Over a period of at least a year, maybe more, it was getting slower and slower. Performance was modest (but adequate) to start with, but it slowly deteriorated until on a level motorway it would not quite hit 70. There was no misfiring of roughness or anything, it just would not pull. It started and ticked over beautifully and would rev in the lower gears but simply had no power.
This crept up on me slowly. I tried changing the air filter, the diesel filter, read the fault codes (none), but nothing made any difference.

I don't use the car much and fuel consumption was still good (roughly 60mpg) so mostly I just put up with it.
But I was revving to 4,000 rpm in every gear just to pull away up hill.

I tried unbolting the exhaust from the turbo to see if it was a blocked CAT - no difference.
Turbo was not blocked and seemed to be spinning fine.

Then the dash started flashing EAC Fail! occasionally. It made no difference to performance. Turning ignition off and on again would clear this. Still no fault codes. Then EAC fail after the first mile of every trip.

I decided to bite the bullet and take it to a garage. I am always wary of this because they all charge a lot and in the past I have had experiences where they have simply thrown money at the problem (my money) until they get lucky and replace the part that is faulty.

Anyway, they suspected the MAF sensor, had one in stock, replaced it and the car is now back to normal. I feel like I'm driving a rocket ship! That will wear off of course.
The garage charged me about £245 which was 2 hours labour (one hour diagnostics and one hour fitting)  and £125 for the part.
See what I mean about charging a lot?! I think I could have got a MAF sensor for about £30 and it takes 5 minutes to fit.

I tell myself I am not paying them for the part, I am not even paying them to fit the part, I am paying them to know what part to fit. 
And at least they went straight to the right part to fit.

So now I'm telling you all this story, because if I had read a post like this, I would probably have taken a punt on changing the MAF sensor, and saved myself a lot of money.

Had exactly same problem myself on my 2005 1.4tdci - it's in previous posts.... eac fail can be thrown up by MAF sensor, £50 from euro car parts, cannot remember the brand, there was a cheaper one but after research it was advised to go with the brand i did.

 

What threw me in my case is that at the same time i had what i thought was an alternator fault - which then was revealed to be the 3 wire control plug to alternator being badly corroded causing the battery to go flat. This was resolved, then eac fail kept reappearing then i resolved that with maf sensor replacement.

I never had a problem with the maf sensor until the flat battery issue occurred strangely.

The foot throttle potentiometer works as voltage divider with +12v at one side and 0v (gnd at the other, the slider of the pedal works inbetween the two states - if the alternator fails and drops below 12v the throttle slider then sends a voltage too low and the ecu see's the error throwing eac fail.

 

Iroinically the alternator failed on the stator winding 8 months later, and thats another story....