A question about the alternator. If the headlights flicker when changing gear and allowing the revs to drop is it the regulator that is at fault?
It was one of the reasons i stopped driving my 406, the battery charge fault came on.
My only experience of a regulator packing up was silly amounts of voltage going into the electrics (headlights got bright, wipers went crazy). I changed the regulator on that one but that was on a 1980 VW (yes, once I was lost...) and I'm not sure how rebuild-able these things are now.
I had the same issue with my 406 - whenever revs dropped, the lights dimmed ever so slightly. But only when the car was cold - seemed OK once warmed up (presumably the longer you drive, the more charged up the battery is). Never really got to the bottom of it - even a new alternator hadn't fixed but was too little an issue to worry about.
I replaced the brushes and slip rings on the 807's alternator. They are far more likely to be your issue, since they're the only parts subject to wear and tear.
IIRC, it cost me about a tenner for the bits and I found a guide on Yewtoob from one of your countrymen, which was very informative.
(I'd never have figured out how to strip it down without breaking it otherwise).
2002 HDi 2.2 Exec Estate, (2008-12) (wonderful)
2003 HDi 2.2 6-speed Exec Estate (2012-19) (also a gem)
2009 Citroen C5 2.0 HDi VTR+ Estate (godawful heap)
2008 BMW E91 330i touring (great fun - murdered by a reversing SUV)
2007 BMW E91 325i touring (slower smoother quieter)