I remember getting a Peugeot 307 HDi loan car, all was well until i needed to go faster, it was coughing and spluttering. So i drove on a bit further and stopped. Thinking that its a loan car i smelt inside the fuel cap, it was petrol.
I chugged it back to the dealer who didnt seem too bothered, they then gave me a Kia Cerato (i wouldnt care but i didnt even put the fuel in and i got this as punishment!). When i went back to collect our car i asked about the 307, they just said, oh yes, really chugs like a good'en that doesnt it.
Thats why you never buy an ex loan car
Often though, unless the car is driven until it stops, there is no immediate damage. Though on these new high pressure diesels im not sure what problems misfueling could cause in the future.
A 1.9 Xantia should not think twice about it though, they dont build em like they used to!