A garage should be able to tell you whether the system is charging or not. Do you do a lot of short journeys? Where you're depleting more than you're putting back?
If the battery is holding charge. I'd check the pulley on the Alternator, they can become stuck, it should freewheel one way and lock another.... Open ended spanner on the idler pull towards you and lockout with an Allen key through slot, remove belt (inspect and put mark for direction), disconnect battery (do this first actually, lol), place something into alternator to stop fins from spinning, and turn the pulley, as above it should lock one way and freewheel the other, if its locking both ways or freewheeling both ways, it needs replacing.
An impact gun is helpful for removing the pulley but not totally necessary. You can get refurbished Alternators on ebay with a new pulley on, refitting the alternator isn't too bad, just a bit access limited.
