chance_favours wrote:
Your alt. isn't charging.
Should be around 14.2, with a load ie. headlights on, and around 1500rpm.
Once alt. fault has been fixed, give the battery a long charge overnight on a good charger and hopefully it recovers to full health.
+1
You can charge your battery before you get around to fixing the alternator. This can also prove the battery is fine. Charge it up for as long as it takes to get full voltage and test to see it turns over and starts fine.
Shut it down and recharge to full and leave it as it is for a few hours or overnight, the battery should hold its voltage and start up normally. If the battery is good, and the alternator dead, you should be able to get to where you need to go locally (like work for instance) and recharge it while you go about your day herding cats, or stuffing goats or whatever you Shepperton people do up there

I've been able to drive for better than an hour without a working alt, the battery has the capacity to run on the battery alone for quite some time. Maybe don't push it under headlights however.
On the low voltage front, batteries are designed to have a certain voltage, naturally around 12 volts for a car battery. If you drain the battery by cranking it when it is flat then naturally the battery will drop to a low value . Once you let the key go, then the battery will crawl back up to as close as it can to its nominal charge value, even though there's nothing in the battery to back the voltage claim up. Different batteries do different things as the battery charge decreases. some are more linear as they get to zero as you would expect. Others hold there voltage better while having little charge left. Lead acid batteries are like this.