My Cambridge interview tested for this explicitly. They gave you 45 minutes to prepare answers to 8 'further maths' type questions, then you had to solve some creative problems at the end. These creative problems required knowledge of further maths concepts but were unusual so you needed time to think about them and develop a mathematical framework for the solution, this therefore tested that you could do the further maths questions easily and quickly and that you could solve new problems.
The 'creative' problem I had was this: A mouse walks along an elastic band at 2 meters per minute. The elastic band stretches by 1 kilometer every hour. How far along the band does the mouse get? If you got that right, then they gave you a stretch question where the band doubles in length every hour.