Ciaran wrote ...

It's quite feasible for unexplained glitchiness and particularly things like failure to start, dying suddenly, ancillaries like indicators not working, to be caused by a battery that isn't up to par.
The likes of the XM are very hard on their batteries, but the C6 and other cars of the modern CAN generation take it to a whole new level. There are a million and one computers, core functions controlled and modulated by software, plenty of scope for error and unexpected behaviour if battery supply isn't optimal.
On older cars things like the indicators were physical, a switch, a power source (battery), a circuit and an output device, such as a bulb at the other end.

With the C6 and cars of its ilk, these functions and devices are nearly all logical. When you operate the stalk, the various ECUs communicate, via the BSI, and a 'request' for indicator flash is sent through the relevant CAN networks, eventually, hopefully, resulting in the bulb flash.
You can see a lot of this terminology when browsing the various ECU functions in Lexia / Diagbox, things like 'Request for incicators', 'Request for horn' etc.

If the battery is not providing ample supply then plenty can go wrong with this convoluted process and its quite common for things to just stop working altogether, as you saw with all your 'failure to communicate' with the BSI, nevermind physical operations such as turning the starter motor.

To be fair, these issues are by no means unique to the C6, any modern car is likely to suffer similar issues if the battery isn't 100%.

jamescarruthers wrote ...

Is that the same for the V6 petrol? I'd like to check mine as the garage changed it jut before I bought it. Everything else they did was wrong!