Mdon wrote:how hard are they to replace?
It's a job that
looks difficult but is in fact dead easy

It's rare on a Xantia to be able to say that but for once it's true...
Jack up and support the front of the car, release the cables from the calipers, crawl under, release the edge of the exhaust heatshield on the side where the cables disappear into it, prop the headshield with a block of wood, slide your hand in and feel for the cable compensator where the two cables terminate. identify the nippe of the one you want to replace, flick it up with your finger to disengage it from the compensator and pull the cable free.
Install new cable, taking care to get it through all the guide rings and then offer it back into the compensator with the inner fully extended, push and the nipple will engage into the mechanism.
Connect the other end to the caliper and adjust as per the BoL.
Note the cables are 'handed' in regard to length and the big engined cars (2.1TD, 110HDi, V6 and Activa) use a different LH cable to the rest of the range. GSF don't do the big engine LH cable but good factors can supply a Q-H cable at a good saving to the genuine Citroen item.