Stick the meter across the battery and check the voltage output of the alternator. It might be a duff one. If its output is erratic, check the brushes are free to move before condemning it. If it has been stored for a while they may just be a bit sticky.
Battery measures normal, well at least it did when I measured. When I asked my two youngest to do it it went up and down wildly but I'm hoping it was just their little not so steady hands.
Right will need to check this at a later time. I checked the cables seem to be on tight but need the car up on ramps to check properly tomorrowSeems very likely to be a dicky contact. As the engine vibrates, the contact is moving, so the earth wire to the engine seems the first place to look. If it is covered, it might have become brittle inside, and is making/breaking as the engine moves. Also need to check the power cable from battery to alternator to ensure tight. If this is fixed to the alternator with a nut, ensure the post in the alternator is also tight.