Battery or computer going nuts

[Johnny C]

Hi. I drove 90 miles. Stopped. Engine wouldnt turn over, message screen said all sorts of things wouldn't work. Got a call out from a mechanic, jump started the car. Ran for 10 mins to get me half a mile to where I wanted to be. I could jump start from there to get home. Alternator is pushing 12.3-14.7. After 10 mins on jump start 3 days later tried starting engine worked straightaway. Has worked fine ever since. It's not the battery

 

Is it the computer? If so can I reboot iir? 

[Magnet]

How do you know it’s not the battery John? 

[Johnny C]

Because it's now working fine. Driven 100 miles today started fine multiple times. After it went 'flatt' was charged enough to drive 10 mins. Started fine 3 days later. 

[Magnet]

Good, but back to your question - why wasn’t it performing its function when it let you down? 
The only way you are going to get proof positive that it is fully serviceable is to get it tested. Halfords stores will do that. 
Get the alternator output check at the same time, since 12.3 volts is way too low. 
Until you get both these checked, sorry, but you cannot eliminate either as being a culprit.