Neighbours C5 has jacked the rear up really high and stuck there -
> likely to be anything obvious and easy to fix or is it yet more
> french quirkiness?
The standard answer on Xantias, BXs and the like was the rear height sensor. There was a plastic clip which connects the suspension to a valve which controls the rear height. The plastic clip can break or the valve can stick. Unfortunately I've given away my Xantia Haynes BoL and never had this trouble on any of my Cits so I can't help much more.
I'll cross post this to alt.autos.citroen.