It is possible but not something I would tackle. (I have been a mechanic for 26 years).
There are 2 types of clutches on the Xantia, push type & pull type, the pull type is the most akward.
You will need a second person to help when doing the job on the floor, or will need to have the vehicle high enough off the ground to allow the gearbox to be supported by a jack. This is especially true of the pull type clutch. On refitting of the gearbox the clutch fork needs to be hooked behind the release bearing.
Good luck, but in my mind £400 is a small price to pay to save the aggro. (remember if you start the job and get to a point where you get stuck with the gearbox half out and are unable to finish the job, your only option then is to have the vehicle recovered costing more money)
David Scruton
June 2008