Noticed that when reversing into a parking bay, for example, when the brakes are applied a single 'knock' comes from the front brakes when they're appled.

 

No equivalent sound is heard when the brakes are applied and the car is moving forwards.

 

Any ideas?

Pads moving in the carriers.

As Rover says, almost certainly pads moving in the calipers. Perfectly normal, especially if aftermarket pads have been fitted as quite often they can be a noisier fit. If it gets worse then get it checked as it will be something else.

3rded, or possibly fourthed. Anyway, it was my first thought on reading the title in "recent topics", without actually knowing the model or trim level.