Sienna Miller's 'secret' for younger-looking skin is just £13 at Amazon

Whenever I see Sienna Miller I question what's in her water, because (while there's nothing wrong with ageing) there's no way she looks 42-years-old.

The actress and model is arguably a style icon first, but her beauty looks are also perfection, and she's just shared the niche (and affordable) French pharmacy product that keeps her skin smooth and glowing.

Speaking to Harper's Bazaar for their 'Inside my beauty bag' series, Sienna revealed she uses the A313 Vitamin A Pommade by Pharma Développement - a retinol cream - which is available to shop at Amazon. And if you thought it would break the bank you'd be wrong, as it's just dropped in the sale for £13.35.

A313 Vitamin A Pommade

"As you can see by the state of my beauty bag, this is genuinely my beauty bag" Sienna told Harper's Bazaar. "I haven't really been through it, I have just been away for a month and I grabbed it so this is authentic.

"This is a really fantastic secret. It's called A313 and it's a French retinol that you can get over the counter. Alexa Chung told me about it and she's got the most amazing skin. It's described as the French girl secret to great skin, so we all need that."

© Craig Barritt

Retinol (also known as Vitamin A) is most widely used to prevent the visible signs of ageing, so anyone over the age of 30 and interested in skincare is likely already using one, but choosing from different strengths and formulas can be a bit of a minefield.

A313 is on the more gentle side, containing retinyl palmitate, which is a combination of retinol and the fatty acid palmitic acid. It's also formulated with two types of polyethylene glycol to stabalise the retinol and help it to penetrate more efficiently, while its antioxidant properties work to protect your skin's barrier.

It's designed to be applied to dry skin at night, before your moisturiser, and you only need a very small amount - a little goes a long way.

But don't just take Sienna's word for it, online reviews also suggest it works wonders on smoothing your skin, as well as helping to reduce pigmentation and tackling acne.

"I've been using it for almost two months now as directed by another review," wrote one Amazon customer. "Twice a week, then three times a week, then every other day and then every day. Each for two weeks at a time. In the beginning my skin purged like crazy, it made my acne so much worse. That lasted a good few weeks, but got less and less as time progressed. Now I finally have clear skin, the first time in over 15 years! I haven't had skin like this since before I hit puberty!"

Another said: "Last year I searched every pharmacy in Paris that I came across only to find out the product had stopped production for the time being. Which made me want A313 even more. This cream really does work. You only need a little on super dry skin twice a week but the results can be seen the next day. Smoother and brighter skin."

And another added: "I've been looking for a good retinol but without the expensive price tag and this is the ONE. I use this every other night at the minute along with retimax on the nights off, and my skin has never felt and looked so good. I will be repurchasing."