It's apparently something to do with the brake drum the contents of which appear to have fallen off. After a 16 mile journey to the dealership the noise was a lot worse the drum was red hot.
I'll post the final diagnosis when I have it.
The Clanking sound turned out to be the End Float play in the bearing to hub fit. This required the replacement of the hub and bearings. The rear wheel was almost seized and the wheel cylinder was also replaced.
Thanks to Mike at Allon White for the excellent service and Phil for honouring the repair when just out of warranty.
