Recently, when reversing out of my drive and then turning the steering wheel on (almost) full lock, I 've heard a 'clunking' sound, similar to a spring being twisted. I haven't looked underneath as yet.
Any idea what could be causing this?

Thanks.

Broken spring?

get the front diff / PTU checked. It may not be that but its possible

I had this on my first Evoque, loud clunk from the rear nearside wheel and turned out to be the PTU had stripped the splines to the gearbox.
Think a few others on here have had similar issues and diagnosis too.

If in doubt message Austen at Bell Engineering who is the recognised drivetrain expert.