I had to call the RAC out as my car wouldn’t start (was at home) and I got the Bong Bong Bong sound followed by “ESP fault on the MFD”. Pus the engine wouldn't even turn over. Guessed it was probably the battery but called out the RAC anyway.(Got to get your money’s worth)
He arrived and quickly diagnosed it was the battery, as he said the first things to show faults with from a flat battery are the ESP and air bags! He got the car started ok and tested the battery, with his hand held tester, and it came up “Battery fault, change battery.”
He then offered to fit a new battery for £100. Which I politely refused, using words I think like F**K off… and robbing B*****D.

Contacted dealer the same day and dropped the car off. The important thing to note here is that they will change out the battery under warranty, but the head office stand point to all service departments, is that they must do full charge and discharge tests, cold starts etc to prove it is faulty,
which will take on average 4 days. 
Dropped the car off on the Friday got it back on the Wednesday. (NB. This was before Christmas)