Got the red Battery light up on the dash while on the commute into the office. Happened while in stop-start traffic and no strange noises from the engine area. Aux belt intact and engine runs fine. This points to a fault with the Alternator. A multi-meter across the battery confirms with the engine, showing 11.6v (should be 14.4v), confirms that the alternator is not charging the battery.
To fix this, I would need either a new alternator (second hand unknown units from 40€ upwards) or repair this one. I found repair kits on ebay with regulator, bearing, brushes and slipring and from a supplier in Mayo, goldlion_autoelectrics. Repairing the existing alternator with brand new parts, which is in great condition otherwise, is more appealing than replacing with a cheap second hand one that may itself fail who knows when.
In the end, just replaced the regulator. There was loads of life left on the brushes. Put back onto the car and all working great again.