Alternator Fault, Battery Not Being Charged!

[P Smiddy]

Again I need to pick the brains of experts.

History

Last month I got the message in the title on the dash, my solution was to turn off the car leave it for a few minutes then start it back and and presto, it was away.
Planning to get the car into the garage and get it looked at in the new year. Today it happened again but this time when the car was running and I was stopped at lights it idled slightly higher than usual.
Since i was about to go onto a ferry i dedcided to turn the car off and on again to clear it again incase i get stuck on the ferry and it was a good desision! Car would not start, totaly dead and wouldnt attempt to turn over so called the AA out and literally just before the van arrived I thought I would try once more and with no fuss it started as if there was never anything wrong, kept it running and Mr AA done his checks.
Results - Altinator fine and battery weak. He said it is known for the alternator to be intermittent and advised me to get both changed.

My question is, Alternator I thought it either worked or it didn't and wasn't a case of it being intermittent. Should I replace both?

Thanks, Excuse spelling mistakes......trying to write this on the tablet whilst being a passenger.

[fat controller]

I'd be inclined to blame the battery to be honest; I am no mechanic, but and reasonably savvy when it comes to electronics, and I know for sure that our cars are VERY picky when it comes to having a battery in good health.

To be sure, get a VCDS scan done and see if it shows anything. Failing that, a trip to the dealer for a new battery and alternator health check is on the cards.