So after much back and forth with my local main dealer and the supplying dealer, I ended up taking it back to the supplying dealer, as the local main dealer were basically saying it is not covered under Das Welt and "who was going to pay for it". Supplying dealer is 100 mile round trip, and they said they would speak to my local main dealer, and confirm it would be covered by them, but the more I talked to them, the more it sounded like a potential for a f*ck up. So I did a deal that if I bring it back to them, they would return the car to me with full tank of fuel, as they wanted the car for a few days.
So they had the car back and flushed the coolant system, replaced coolant bottle with a brand new one and refilled with new coolant. They said they couldn't find a fault. Someone suggested it might of been the previous owner topping up with blue coolant and not pink, and them mixing together, who knows? Still odd whatever it was, wasn't spotted on the bulls!t Das Welt check. I am going to keep an eye on it and will report back if it reappears.