Could only fine a couple of references to parking brake, so resurrected this one.
Have had an annoying rattle from what sounded like the fridge area, very metallic and was getting more consistent. Did all the usual checks of draws, stuff in draws, cupboards etc but couldn't find anything. Then whilst parking, and the auto parking engaging, there was a small bang and the warning as per the picture above came on.
As under extended warranty, phoned for assistance and the RAC attended. It is a snapped parking brake cable. The car is safe to drive, and now just uses the auto box park. Phoned a dealer and they performed true to form - £139 just to have a look, so went to an approved garage. Still waiting for the quote but it might be a bigger job than just a cable replacement as the motor might be damaged. Intend to check with the warranty as a cable is (not that i've ever had it) wear and tear, but not the motor in my opinion.
Whilst driving it without the cable the rattle has stopped, so that was it, a loose parking brake cable hitting the underside of the car. I'll post the outcome of the fix on here. Will be of interest to those with MB extended warranty.