So a little background - Mercedes C Class Coupe 2016 Auto Diesel (done 12k miles) bought privately about 6 weeks ago. It had a Mercedes service just before I purchased it. Over the last few weeks the auto stop/start function rarely ever turned on green but I didn't think much of it. Then a few days ago it started working a lot more often BUT then at a set of traffic lights when I tried to move the car can't turn itself on I can hear it clicking and trying to turn on but it can't. I gave it a minute or two and after a few attempts it turns on. Later that day I drive again and the auto stop/start is on and after stopping it completely fails to turn on at all. I call my breakdown assistance they come and using jump leads turn the car on and tell me they think its the battery. I've taken it to a garage who have done a battery health check and have told me the battery is completely fine and that there must be something else causing the car not to turn on at all (unless using jump leads) and that I should take it into Mercedes. I have booked it into Mercedes next week (but I'm sure everyone is aware how much that will cost). Does anyone have any ideas what it could be or what I can do at all? I'm going completely besides myself having only bought it a few weeks ago and now unable to use it at all..

Hugely appreciate any help and advice!

Thank you!