The French have never been good at luxury cars. Citroën is about to make its last C6. The DS of 1956 was a phenomenon, but its hydropneumatic circuitry and aircraft-quality technology didn’t work in cars. I once had a Citroën collapse embarrassingly on a Scottish press launch.

About to take off from Heathrow, my British Midland Boeing 737 taxied back to its stand. "We will not be going to Glasgow on this aircraft," the captain apologised. "We have an instrument warning light, which forbids us taking off." He said it was tiresome, probably nothing, but rules were rules and would everybody please get off.

Passengers agreed warning lights were not to be ignored, and we set off in another aircraft to try Citroën's reply to the BMW 5-Series, a 24valve XM. The warning light was apparently connected to the aircraft’s hydraulics. It was not going to be a good day for hydraulics.

Oily fluid in pipes conveying pressure, pumping up air springs, gave the XM a floaty, matchless ride. Electronic sensors puffed up air bags working in conjunction with the hydraulics, keeping the car on an even keel the way Citroën invented in the 1950s. You barely gave hydraulic brakes and hydraulically-assisted steering a thought; they had been with us so long and there were always mechanical alternatives on which to fall back - steering by rack and pinion, a rod-and-cable handbrake.

The flagship Car of the Year XM had a V6 24-valve engine of 200 horse power, did 145mph, and reached 60mph in 7.5sec. It was well-proportioned, had alloy wheels, colour-matched bumpers, and luxury appointments such as electric seat adjustment, air conditioning and wood door cappings.

Half way through the test the power steering stiffened. On a couple of corners hydraulic assistance left off assisting, and the light feel grew leaden. When it gave up altogether it was just like driving a Citroën with very heavy steering; the car went where it was pointed and it was satisfying to know that when steering hydraulics failed, there was a mechanical system to take its place.

However, a Citroën's hydraulics failure was problematical. They were central its operation, and some 30 miles further on, the suspension sagged. A warning light came on and a buzzer sounded. The facia said STOP! With an exclamation mark. Citroën's hydraulics had stopped hydraulicking. This warning light, unlike the Boeing’s, meant what it said.

A drive belt, the Citroën PR man claimed, had come off. The pump for the hydraulic pressure was pumping no more and the car decided that, although most things were still functioning, pressing on with the suspension in a state of collapse was not an option. Citroën found me a replacement; we were air-and-fluid-borne once more, completing the journey through the Highlands in the serene comfort of a top Mercedes-Benz or Rolls-Royce.

The 1956 DS had tempted buyers away from Mercedes-Benz, but in the 21st century Citroën finds itself unable to make the C6 compete with premium brands, principally German. Only 556 were sold in Europe in the first 10 months of this year, so it has stopped. Peugeot also has quit the large luxury market so French consumers looking for premium saloons will need to buy BMW, Audi, Mercedes-Benz or, quelle horreur, a Jaguar. Renault makes the Latitude, but buyers in France are not buying it because it is a rebadge of the Samsung SM5, imported from South Korea. Up to October, Renault sold 3,792 Latitudes, a decline of almost 60 percent from last year.

Renault might develop a new flagship with the underpinnings of a Mercedes E-class, selling as the Initiale Paris sub-brand. Automotive News Europe is not alone in wondering whether Renault should follow Peugeot and Citroën and leave the big luxury segment to the BMW 5-series (101,600 sales in Europe after 10 months); the Audi A6 (89,300); and the Mercedes-Benz E-class (86,400). In Europe, this market segment speaks principally German. The end of the rainbow for the C6.