French dressing: France's first lady arrives in Britain

When Carla Bruni-Sarkozy came down the steps of the plane at Heathrow yesterday, you could be forgiven for thinking one of three things. That she had swapped outfits with an air hostess on board, that she had spent weeks studying pictures of Jackie Kennedy and her classic pillbox, or she had renounced being the President's wife and taken holy orders. Or perhaps all three.

For the supermodel turned singer and serial rock-star girlfriend who has declared herself easily "bored with monogamy", it was an astonishingly demure, almost nun-like outfit – but then that was precisely the idea. With a bold, nude picture of her just released to the press, it is hardly surprising that Mme Bruni-Sarkozy wanted to cover herself up as chastely as possible and present an image that was formal, decorous and appropriate – a world away from the bling-bling president label that has attached itself to M. Sarkozy and his Ray-Bans.

It was a chic, flattering shape, and showed off her figure but the grey colour was so understated she almost blended into the asphalt. Or perhaps she had matched her clothes to what she had heard about the British weather. Strangely, she changed three times throughout the day, but into variations on the same demure theme. Even at a white-tie state dinner in the evening she eschewed high octane red carpet glamour in favour of a navy silk floor-length dress with delicate jewellery. The sheer cape on the dress ensured that her arms were largely covered, and she exposed nothing more daring than a flash of forearm.

But perhaps these ensembles should not have come as such a surprise. Her dress sense is nowhere near so wild as her bohemian past and romantic trysts might lead you to expect. In fact, Mme Bruni-Sarkozy's recent outfits have been fairly understated. She often favours white shirts, plain black outfits or jeans and simple jumpers, almost always with flat shoes.

Yesterday's outfit was teamed with flat ballet pumps, while it was M. Sarkozy who sported shoes with a considerable heel to even out the height difference between him and his beloved.

Mme Bruni-Sarkozy has not just toed the line with her tactful footwear and high neckline; her sober wool and jersey belted coat, bag and gloves were all made by Christian Dior, as were the two outfits she wore at Windsor Castle and Parliament.

By choosing a Parisian designer she has shown support for France and one of its key industries – unlike Cécilia Sarkozy, who wore her rebellious streak on her sleeve when she wore a dress by Prada – an Italian label – at M. Sarkozy's inauguration.

M. Sarkozy might be branding himself as forward thinking, but it is his wife who has her eye on fashion's future. Her clothes – from Dior's autumn 2008 collection – will not be available in the shops for a couple of months.