What IS Sienna Miller's secret? Incredible renaissance for actress, 42, as she looks better than ever just four months after giving birth - after crediting yoga and Marmite as the secret to her age-defying looks

Sienna Miller stunned fans as she showcased her incredible figure in a £3,900 gown at a star-studded birthday party, just four months after giving birth to her second child. 

The fashionista, 42, and her boyfriend Oli Green, 27, welcomed their first child together, a baby girl, in January. Sienna also has a daughter Marlowe, 10. 

The actress has barely aged a day since starring in Layer Cake and Alfie more than twenty years ago - and previously credited yoga and her love of Marmite as the secret to her age-defying looks. 

Sienna joined the likes of make-up artist Charlotte Tilbury and models Poppy Delevingne and Cora Corre at Lady Lola Bute's 25th birthday party, held at the family's Mount Stuart House on the Isle of Bute. 

Sienna stood out from the crowd in a £3,900 floor length silk mesh dress created by Annie Doble of Annie's Ibiza. 

Sienna Miller stunned fans as she showcased her incredible figure in a £3,900 gown at a star-studded birthday party, just four months after giving birth to her second child

Sienna stood out from the crowd in a £3,900 floor length silk mesh dress created by Annie Doble of Annie's Ibiza (pictured with Annie Doble and Camille Charriere) 

It featured hand-beaded crystals in the shape of a cobweb and a backless design, while the sheer nature of the dress highlighted her lithe figure. 

She teamed the look with a nude bodysuit and a bronzed tan, having just got back from a family holiday in Croatia. 

The designer also produced a custom-made outfit in a 1920s Egyptian fabric for Lady Lola. 

As Lola and Annie shared photos of the bash on Instagram, Sienna firmly cemented her status as a fashion icon as she posed for a photo with Annie and Camille Charriere.

Sienna previously gave an insight into her health and fitness regime and told Vogue: 'I do yoga three times a week…more because I’m just searching for some equanimity. It’s very easy to feel exhausted by these schedules and time zones.'

The actress, who lives in New York, said at the time she wasn't a big fan of the spin class SoulCycle. 

She said: 'Sometimes I cringe a little bit at the motivational aspect of it, but that’s just because I’m English and we’re not used to saying, "You see your dream! Go get it"'. 

'Cooking is a focus for me,' Sienna shared, 'But I wouldn’t say healthy … I’m more of a home cook: roasts, some Asian food, pastas and sauces and soups. In terms of, like, chia seed and whatever grains, no. I’m like, "Fried eggs and Marmite toast".'

The actress looked incredible just two weeks ago as she holidayed in Croatia 

The fashionista, 42, and her boyfriend Oli Green, 27, welcomed their first child together, a baby girl, in January

The actress has barely aged a day since starring in Layer Cake and Alfie more than twenty years ago (pictured in Layer Cake in 2004) 

The party saw hundreds of guests flown out to the remote Scottish island for the party of aristocrat Lady Lola including Sienna Miller (pictured) 

Charlotte Tilbury was among the well-heeled guests enjoying the festivities 

Lady Lola Bute celebrated her 25th birthday in style in her ancestral home this week

Guests shared snaps inside the home, including in the bedrooms they were saying in

The party saw hundreds of guests flown out to the remote Scottish island for the party of aristocrat Lady Lola, who is the daughter of the late John Crichton-Stuart, 7th Marquess of Bute and fashion designer Serena Bute.

Guests had a weekend-long celebration, dancing in the 19th Century Gothic revival dining hall, and sleeping in grand bedrooms that look straight out of a fantasy novel.

Princess Olympia of Greece and Lady Mary Charteris, danced with members of the Guinness family, who downed Iron Bru  during a festival-themed party in the day before switching to more sophisticated dirty martinis with their black tie attire in the evening.

Rare snaps from inside the Mount Stuart House were also shared, including a grand staircase, ornate ceiling pattern and even the somewhat old-fashioned toilet.

Sienna showed off her baby bump at the Vogue World in December, before giving birth a month later to a baby girl 

The manor, which is the seat of the Marquess of Bute, a title now held by Lola's brother,  has been held since 1157. 

However, the current home, which is Category A listed, was built in the late 1870s, replacing an earlier house.

This weekend however, the home got a huge makeover, with giant inflatable mushrooms and teepee as part of the party set-up.

After partying the night away, guests also enjoyed time on the island, heading out to sea and wondering along the beach.

Keeping with the Scottish theme, Lola also enjoyed a bagpipe band, and drummers in kilts.

The party was organised by Lola's half-sister, Jazzy De Lisser. 

Jazzy is the daughter of Serena and her first husband Robert De Lisser.

Despite her party girl antics, Lola is sober, having sought help for 'unhealthy coping mechanisms' following the suicide of her boyfriend Kai Schachter and the accidental overdose of her best friend Ila Scheckter. 

After Kai's death, she founded charity Eternity to bring attention to suicide, addiction and mental health which she said helped her to 'heal and cope with grief in a healthy way'.

Her father, John Crichton-Stuart spent much of his time on his namesake Scottish island at his ancestral home, Mount Stuart, and had chaired the Board of Mount Stuart Trust since 2005.

The 7th Marquess, once ranked 616th in the Rich List, was also a Formula 1 driver in 1986 as he partnered Ayrton Senna and raced under the name Johnny Dumfries.