What engine are we talking about here? The V6 doesn't have a turbo, does it?The TD4 is a great engine, and for uncharacteristic loss of power, rough running and unresponsiveness, people are right to point you towards split turbos, faulty MAF sensors (the give away is that when plugged into a testbook computer, a failed MAF gives no indication of a fault) etc. It's also worth checking the cam sensors in these cases. www.tuning-diesels.com is an odd looking website but full of useful tidbits about the TD4 (and the earlier L-series diesel) from "Roverron".
Nevertheless, the "revving but no increase in speed" sounds clutchy, doesn't it?
Cheers
Blippie