I'm a new member and my experience is with the late 1970's R-5's sold in the USA market, but I have 20 years' experience working on these cars and can suggest the following:
There are two most likely causes of the warning lamp flickering for the battery indicator, labeled ALT on some vehicles.
First, the most likely: There is a single pin coming out the rear of the alternator which a (usually) small gray wire attaches to by pushing on. This is connected to the printed circuit board underneath a cover at that point. This pin is the sensor output for alternator voltage. The pin is stainless steel or some similar material and is staked onto the circuit board. With age and vibration, the pin makes poor contact with the board, and since it is stainless it does not take solder. This makes the warning lamp flicker on at random times. I permanently solved this issue by removing the board and soldering a flexible wire from the foil around the pin joint on the foil side of the board, over the edge to the side the pin protrudes from, and wrapping it around the pin, then soldering that wrap. This eliminates the bad contact issues. In USA market cars, this output was also wired to the "broken belt relay" which would illuminate all the other warning lamps as well.
The next possibility is that the alternator brushes are worn down, if there are a lot of miles on it. Located on the top of the alternator under a plastic cover, when worn they will make intermittent contact. You can test this if the light is on steadily or flickering, by pressing down on the plastic cover. As I recall, the brushes can be changed without major disassembly of the alternator. In an emergency you can stretch the springs a bit to get a bit more life from the brush set. At one time the brushes were a standard service part and inexpensive.