All the Citroens i have owned up to my current Hydractive Xantia TD have been petrols....the problems i had were numerous. They seemed to like blowing head gaskets....
Touch wood, my current Xantia is performing almost faultlessly. It does have a couple of niggles but thats down to its history of patchy repairs, nothing else. The Diesel is absolutely king 8)
Fuel economy is great, the noise intrusion is very low, it has plenty of poke in comparison to the petrol Xantias i have owned (1.8 and 1.6).
Ive also had a 2.1TD 406. But like Jim said "once you've had a hydraulic Citroen there is no comparison" Never a truer word has been said.
What garage has the V6? I know a few places up near Blackburn, one in particular i know sells alot of Xantias but are known to be a bit dodgy