Charging problem

My MG ZT-T diesel is not charging properly, noticed the battery was going flat and when trying to start just clicking, my immediate thoughts are it was a faulty battery so jst to make sure started the car and put a voltmeter across terminals for readings. The charging voltage at idle was onlt 12.4v, so i wired in a voltmeter to measure the voltage whilst driving and with fan, lights etc turned on. The maximum it charges at is 12.9v with nothing on, with sidelights it is around 12.7v with heater fan blowing 12.2v. Now it is obviously an alternator fault but any ideas what? rectifier, regulator? Best part is the wife has exactly the same problem on her Ford Focus but I am in the process of rebuilding an alternator for that as I have a spare.

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