I was getting similar problems with a 2001 Astra 1.7 DTI Y17DT. Eventually diagnosed via opcom to a failing MAF sensor.

The MAF sensor had eventually thrown a code after many miles of falling acceleration. Its voltage was all over the place at idle, going from 0.92 to 1.88, and fluctuated similarly up to WOT.

I cleaned the MAF to no avail, just the same. It never reposted another failure, although it was not working properly.

Purchased item 300850492558 off ebay.

The car now accelerates cleanly and quickly and doesn't have that flat spot.

Check the MAF puts out between 2 and 4 volts, or nearly that, on these diesels, and more importantly check the voltage is steady at any throttle opening within a range of +/- 0.2v.

Typically you won't see a MAF fault until it gets really bad as the ecu will take it for granted.

You must diagnose the car somehow before spending money.

That's what it turned out to be with my car.

Don't replace it just in case. There are too many sensors. You must have a good idea of which one is going down.

Having said that, MAF's a prime candidate, so bet £20 on the favourite.

Hope it fixes it and let us know, post back with success or failure!

@1litregolfeater

You were bang on mate.

Its a failing MAF sensor, the voltage is all over the place

Well chuffed its not a big job so the advice is to obviously replace it then give the van a good blast down the motorway to remove any crap that might have built up in the cat

Thanks for your advice