Given that the belt life is supposed to 100k / 10 years, fork out on a decent timing belt kit - I got an SKF kit for around £55+VAT. Get a new water pump too - otherwise it's bound to fail. You might want to consider changing the thermstat too whilst it's drained down - mine had gone baggy and the running temp had dropped about 10-15°C.
Edit: rmuns gave this link with lots of pics (and French!): http://www.forum-auto.com/marques/Citroën/sujet959.htm
Essential kit: a 10mm flexi head ratchet spanner. At the time of writing Halfords are knocking out a set of 10 (no 16 unfortunately!) for £40. 16mm deep socket required for the engine mount. 22mm socket for the crank pulley.
The process is the usual really:
Remove covers
Remove Aux belt
Remove crank pulley
Remove engine mount
Time engine
Remove belt
Change tensioners and water pump
Refit is a reverse of removal.
Start engine whilst heart is thumping
But here are some tips from having done it:
To get at the aux belt, remove the engine ECU and its box (note that it has a hose connected to the front so needs a bit of a wiggle to get it out). This gives access to get a 15mm spanner on the spring tensioner (I used a bike pedal spanner that is nice and thin), and then you can lift it and pin it with a 4mm pin in the hole behind the tensioner. This will give you enough slack to remove the belt, but you may need to relieve the manual tensioner underneath if you fit a new belt. You might need to unplug the switch on the AC drier to get enough slack in the cables.
Uplug and unbolt the two electrovalves on the inner RH wing, unclip the vac pipes and lay them across the other side of the engine. Put some tape on the bolts or they trash your arm whilst reaching down the engine! I would also tape up the clip on the fuel pipes, as it has 4 sharp prongs that trash the other side of your arm!
Remove the torque link to the inner wing. Getting on the bottom bolt is awkward but it is doable!
There's a 10mm bolt on the top cover down the back. With the valves and torque link removed and your shiny new ratchet spanner this is now 'relatively' easy to spin out. In their wisdom PSA used thread lock on the bolts so they need winding right out. On later post 2000 cars there is a water pipe that further limits access to this bolt.
There's a bolt on the top cover right at the back of the engine mount, but this is gettable when the torque link is removed
The bottom bolt on the top cover (11mm) is actually a water pump bolt, and the pump may start leaking when this is slackened. the top cover is slotted onto this bolt, so it doesn't need removing totally to start with.
The top cover 'slides' in a slot down the back of the engine so needs to be lifted up to get it out, and you may need to pull the engine forward to get it out past the bulkhead.
The front cover is wrapped around the timing belt and can only be removed once the engine mount has been removed. The engine tends to rock back with the mount removed so take care that you are not wrecking the threads on the Torx (T40 I think) bolts holding it on whilst you undo them. There is also some bracketry which fouls this cover and needs unbolting to relieve it.
Crank pulley bolt can be tight, so try and do it with the engine a bit warm, and if possible whack it with a windy gun (my mate's big one fetched it out no bother). Otherwise a long bar, and use an axle stand to support the extension pieces so the socket doesn't come off.
There is a coolant drain on the LH side of the radiator, but it pours all over the wing liner, so it runs out everywhere. Get a hose on it if you can!
The timing point for the cam is at about 7 o'clock - an 8mm bar fits. The timing point for the flywheel is behind the starter. In theory possible to get it with the starter in place, but we unbolted it, dropped it back and down an inch and that meant we could easily get in from above the starter with an 8mm rod in the hole. This would be awkward on your own - the engine is hard to turn over (we used a socket on the driveshaft nut with a big breaker bar - but maybe we could have used a lower gear....)
Make yourself up a tool to fit in the tensioner hole - we filed down a square door bar - the new tensioner had a much smaller hole than the original.
Re-tensioning the belt is pretty awkward - we used one man underneath turning the tensioner, the other nipping the bolt up. Use the tension in the old belt as a reference to set the tension in the new one.
On re-assembly there is a pin on the engine mount that the mount and the plate underneath need to line up with - this takes a bit of a fiddle to make it come right.
Everything else is pretty straightforward re-assembly - but:
Xantia's have an electric water heater in the heater hose, and it seems difficult to get this to bleed properly. I had blown in the header tank and got a load of water out of the heater bleed screw - but after a few minutes idling I noticed the unit was smoking
Presumably had air in it, and had overheated, and was starting to fry the rubber covers on the plugs. I would suggest either pulling the relay for this, or running only a minute or two, and then re-bleeding the matrix connector - or maybe putting a pot under the connector and giving it a really good bleed with the engine running. Unless anyone else knows how to get this to bleed more easily?EDIT: Extreme care is required with this electric heater. It seems that overheating it causes it to leak - both mine and DickieG's that have been overheated have subsequently started leaking. Changing it out looks to be a 'mare, so it's best not to let it cook itself in the first place....
DickieG advises that there are 2 relays in the fuse box behind the battery, each operating 2 plugs - best to pull these till you get it bled, then there's no chance it will overheat
Car seems smoother and more tractable this morning, but that may well be placebo effect!
PSA just cannot design any cam covers without an awkward bolt. It must be a design requisite 
