Side left rear park assist sensor fault - which one is it?

Hi, I have a fault that states "Side left rear park assist sensor faulty". If I clear the fault, it comes right back. If I monitor the live data, as soon as I go in to reverse, this sensor give an error.

But which on is it?

Is it the (silver one in my case) near the rear wheel arch? 

Or it is the left-most black one in the rear black bumper trim?

I'd have guessed the one near the wheel except I have parking aid and I gather the side ones are for that which now has me thinking its the left-most black one. It's apparently easier if its one of the black ones as they are easier to replace and don't need colour coding.

Any ideas?

thanks

Nick