Or you could find the keep alive fuse(s) for the ECU or BCU. Remove and put your ammeter set to 10A range across the socket(s). You might be surprised
ETA: if the battery has been allowed to drain to an incredibly low six volts it will have taken a fair chunk of its life due to sulphation. It might not even take a charge now. Whatever, I would keep quiet about that if looking for a replacement from Halfords.
Note also it's the battery that determines the charge current. The alternator is just regulated to a max voltage - you should be seeing at least 14.4 volts on a near fully charged battery at a fast idle, measured at the battery terminals ideally.
