When a car is MOT tested the actual operation of the ABS isn't tested: what IS tested is the operation of the warning lamp, wether it follows the correct sequence.
2.1 TD's nearly all have ABS-has the light ever worked? The normal problem is that it either stays on or comes on intermittently, so what you have is either a blown bulb, or one that's been removed.........have a look.
It's not all bad news: the ABS on a Xantia is usually pretty easy to fix: the normal causes of the light coming on are either a split reluctor ring on the hub, or more likely a sensor that's picked up a load of swarf and dirt, or a damaged sensor cable: the cables are co-ax and can usually be sorted with a multimeter and a soldering iron, it's almost never a fault with the ECU.
Go to
http://www.frenchcarforum.co.uk/forum/viewforu m.php?f=3 put abs in the search box and you'll find all the problems and all the fixes.