Well…..
Alternator double-checked and all good. All positive leads I can get to cleaned up. It would appear that the X20XEV runs a cable to the starter motor and then a drip lead back to the alternator. The starter motor connection is the only one I have not managed on the positive side.
On the earth, it goes straight from the battery to the bracket behind the PAS pump that holds the aircon pipe. The “stud” looks like it may be the culprit as someone (not me) has had it apart and then done it up allowing the connection to twist right round and bend. All removed, earth connection cleaned up and straightened, along with the stud getting a drop of thread lock to hold it in place (it turned instead of the earth but when trying to undo) and everything reconnected after cleaning.
Whole 0.2V drop now from alternator to battery, no battery light at the moment and only glows slightly with aircon, headlamps, front and rear fogs, heated rear window and both heated seats on, then goes out as soon as heated seats switched off.
Will find out tomorrow for certain. Will also take starter connection apart Saturday when I take car into work to do oil change on the ramps and charge/test battery as access is only from underneath.