Next Saturday night I am going to a white-tie event. In all my years in the dress code Olympics this is a first for me and it's the most formal of them all.

Picture the embassy ball from My Fair Lady. For men it means the full penguin suit, as worn by Fred Astaire; tail coat, stiff white wing-collar shirt with studs, black bow tie, starched white scoop front cotton pique waistcoat, dress pants and patent shoes.

Medals and decorations may be worn, the invitation says. It also generously adds that Messkit (their capitals…), which is along the lines of the red dinner jackets worn during the First World War section of Downtown Abbey, and Highland Dress (kilts with a velvet jacket) are also acceptable. Then it adds – to my husband's intense relief – black tie will be tolerated.

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For ladies, there is not this quarter. For white tie, it must be a long prom dress. It's not as smooth as black tie, for which it is now perfectly acceptable for women to wear a tuxedo pants, a chic short prom dresses or even, as one of my friends recently dazzling, a lame onesie money.

For white tie, ideally, it should not be just a dress, but a structured dress, her arms bare, exposed decollotage - exactly what Blake Lively is wearing in this picture taken during the Cannes Film Festival.

Perfection is a dress by Gucci Premiere, arm seam of the house and unfortunately I do not have very well not have access to this label - or Valentino Couture, which is where I 'd be headed if I had an unlimited budget.

So what am I supposed to wear?

I am fortunate to have a number of excellent vintage shops nearby, so I have obsessive hoping for the perfect dress materializes in one of them. He did not, so in a panic, I bought a long formal gown at the lower end of the price range (it was $ 200, but do not tell anyone, OK?).

It is a kind of Greek-y number of rounds, with a draped, fitted bodice from which falls a ruffled front slit. The best thing is that it is made of heavy navy blue jersey. The fabric should ideally not be allowed too close to an open flame, but it falls very well.

I admit I felt a bit down when I tried it on, as an aunt of the budget of the bride, but I can not justify shelling out money for sheds a unique costume. If I'm going to spend a lot of money, there will be a million to wear ever produced as a cashmere coat or a perfect pair of black pants.

But part inside of me secretly wants to be a princess for a night (the event is one of the oldest colleges of Cambridge - Erasmus he studied - and I think there will be women in real tiaras. ..). Not Eliza Doolittle before she was snatched from the street.

So what to do? Just when I thought I was going to have to make terrifying madness, inspiration struck: I could not gussie my dress cheap with things I already have.

Solve two problems in one, a black lace, short sleeve shirt / jacket covers both my upper arm the danger zone and made me feel less bargain bin, because it's Karl Lagerfeld. I bought it in 1989 I'm so glad I kept it. And I think navy and black is the most chic combination.

Above, to walk instead and make my entrance, I'll wear a fabulous 1920 black velvet soft fur thing. I think it's really a gown, but no one will know. I appear around the neck another vintage treasure, my black marabou cape.

Unlike Ms Lively, I don't have 180 carats of diamonds to sling on, but I will hold my head up proudly in my South Sea pearl necklace and drop earrings and I will – like her – have my hair and nails most beautifully done.

I tried it all on just now, with a pair of sparkly Repetto tango shoes and a vintage evening bag and posed around my bedroom – "How kind of you to let me come…" –- I felt a little like a princess after all.