'if you're about 1/2 hour from home and the alternator fails, what should you do'.
Strikes me that the only reasonable response to that question is "consider all the pertinent facts and then make an informed decision."
I had a similar situation in the US, halfway across a stretch of water, at dusk.
Should I return to the airport I left, 10 mins ago or continue to destination, 20 mins on?
The thought of potentially losing my landing lights and radio (for the pilot operated runway lights) made it a no brainer. The battery did it's job for those 10 mins.
Another day, under different circumstances, may have required a different decision.
I agree with Pilot DAR's citation of the FAR and his conlcusion, but the dilemma for me was that in a rental aircraft, you don't really know what condition the battery is in.