Felix42, it's deffo the Diff, my S1 always did it in France due to the heat.
Change the oil for a good quality 80-140 with LS grade (limited slip) I also used to add a squirt of Friction Modifier & all was good for the next 10k miles or so
If your interested in the tech reason...
Your Roadster has a BTR Australian Axle
It uses cone disks for LS and strong springs set the static pre-load
As the oils break down / thin, eventually could lead to wheel locking under slow corner (hence the graunch or clicking)
