How were you commissioned to create a piece for St Paul's?

London Design Festival got together with St Paul's and, because it's the 300th anniversary of its completion and there is a tradition of doing installations in the Cathedral, it was decided that they'd commission a piece to showcase the architecture, to be unveiled during LDF. I was lucky enough to be asked to design that piece.

What have you tried to achieve with the piece?

I was hesitant about creating an installation. I was hesitant about the word 'installation'. St Paul's is so extraordinary anyway that just to allow people the opportunity to go along and look at it in a different way seemed to me enough of an opportunity.

The architecture of St Paul's is very rich. When you visit you are overwhelmed. It's similar to when you go to a gallery. There is so much to see. So when I visit a museum, I tend to target one painting, but even then, by the time you've got through the rest of the activity and artwork, it can still be difficult to focus on that one thing.

Where will visitors see your piece?

I chose to design a piece for the Dean's Staircase, because it's not well known and yet it's only the second example of a cantilevered staircase in Britain. Inigo Jones produced the first, the Tulip Stairs of the Queen's House in Greenwich. In St Paul's, Wren has designed an exquisite helix of cantilevered stone steps that rises up through the South West tower, also known as the Dean's Staircase and the Geometric Stairs. The engineering that went into creating it is spectacular.

I wanted to create a piece to help people focus on the staircase, as a way of understanding the space. When you look up, you see the underside of the 88 stone steps. It's 50ft up to the top and I wanted people to see how the light from the windows in the tower falls on the steps and how changes in the light through the day affect the space.

So, a highly polished mirror-finished steel hemisphere sits in the well at the foot of the staircase. The optical division of Swarovski, which has always made top-quality lenses for telescopes, made the largest commissionable meniscus lens which sits in the centre of the table, directly below a spherical convex mirror suspended in the tower's cupola. Together these two optical devices create an extraordinary composite image of the view through the stairwell.

It's hard to beat: a gorgeously simple piece. The tower, that is. So I wanted to create a simple piece to reflect the light and proportion of the tower.

What was Wren's magic formula? What could he teach designers today?

Wren had intended to install a Zenith telescope in the tower at St Paul's for the duration of the construction process to measure the Earth's rotation. He knew as much about science and astronomy as he did about architecture. His magic formula was to match his understanding of science and astronomy with architecture and geometry. Wren was a polymath but to get where he got, he would also have needed a deep inner conviction, energy, charm and the communication skills to get it across. He would have put himself in the right place at the right time.

Have you got a design formula? Or philosophy?

Light, geometry, scale, proportion: all of these are essentials in my designs. You have to be uninhibited, but you also have to be sensitive to what's appropriate.

I think I was about fourteen when I realised that architecture which makes you feel something is real architecture. But I was inhibited and I listened to other people who said I needed maths and science, which I wasn't good at at school. So I didn't start training to be an architect until I was 30. The one thing I started with was a passion for doing stuff. The designing bit is what I really love. But design is hard work - the proverbial 99% hard work and 1% inspiration.

You've also worked with Swarovski to create a set of gifts inspired by another example
of Wren's work

Wren was fantastically interested in everything around him. On 26 March 1667 at 4.00pm there was a dramatic hailstorm in London. Wren drew some of the hailstones and later presented the drawings at a lecture to members of the Royal Society.

Looking through the illustrations in the Royal Society's archives, it struck us that the shape of these hailstones was remarkably similar to how diamonds are cut today. They fell in almost perfect condition and the regularity of the faceting is remarkable. Working with Swarovski, we're using Wren's illustrations to create a set of the hailstones in crystal.