Yes, the genuine Seat FR rear mudflap ((575075101A) is a very close fit, you decide if its good enough from the pics below:
Rear mudflap ro body fit.jpg
Rear  bumper with Mudflaps.jpg
 
Interestingly, my original post has been viewed 466 times, but only one reply has been posted (thanks, Cosmic Blue). Whilst several of those views are probably just people being curious, I'm sure some must be from people also wanting to fit mudflaps that fit a recent Cupra Ateca.
Has no one else successfully found a solution?