Good workshop but they do not take responsibility for theft and / damages caused by them

Took my car in for a turbo actuator replacement, they managed to sort it out within a week, only problem is that the battery returned on my car is not mine, I spoke to the manager (Chris), but with no luck, my car started nicely when I took it in, but after collecting it, it needs a jumpstart every time I switch it off. Mind you my battery was 3 months old. Chris does admit that the initial battery was bigger because of the marks and /cleanliness of where the old battery was positioned. But does not want to take responsibility to replace my battery.