battery goes flat

[Niall]

Around that voltage on a HEALTHY battery though

This. If that battery is dead flat, you will find the voltage will be lower until its back to being fully charged. Ideally you will see 14.4v when its fully charged and theres no particular load on the system but typically after starting the car even on a healthy battery, it will drop into the 13s right after starting.
If it was a ford however...