Get someone competent to give the brakes a proper service, ie strip clean inspect lube, which ideally should be done every other year at the latest.

Might just be a sticking caliper piston or slider, after a decent service a few good hard brakes to warm things up and deglaze any dsposits of brake pad material that might be sitting on the discs, course if the discs are scabby and /or worn out then get them and the pads changed whilst stripped.

Did it even show up as an advisory? As Adampr says, it would be noticeable before it became serious. It feels like a slow operating ABS.

If the car passed the MOT brake test I'm not sure how or why they would test for warped disks, there is no road test and the car is stationary throughout the test so unless they took time to set up the measuring equipment for a job which wasn't needed I'm not sure how they would know.

I have experienced a warped disk, which is more than noticeable, slowing from high speeds there was a definate shake through through the steering wheel. The car was used, but had only done 8,000 miles and I noticed it immediately, disks changed under warranty and all was good.

<< With normal driving and occasional derusting with a hammer and old screw driver down the ventilation holes, front discs should last at least 80,000 miles. Ours always have done. >>

In terms of thickness, discs should certainly last that far. What often happens is that most of the movement in the caliper occurs on one side (usually the outer) with hardly any on the hidden side, which fails to polish the disc. Conscientious MoT testers will advise on this, but as long as the brake does its job the car will pass. In time the hidden side of the disc will be so scored that it will need replacing, long before the thickness limit is reached.

Only cure is prevention, by fettling the calipers every 10K miles or so.

The problem can be transfer of pad material onto the hot disc if the brakes are held on after a hard stop or some hard driving (or maybe even normal driving if the rears are providing even less balanced effort than they should for some reason), doubly possible with your Xantia with the park brake operating on the front pads.