Yes, I tried the re-polarising before I tried changing the brushes.
In last-ditch desperation, I took off the generator wiring and the regulator and cleaned all the contacts with emery paper and then petrol.
I cleaned up the regulator mounting screw holes on the side of the wheel arch.
And I squeezed up all the spade connectors.
I put it all back together and after a brief flicker, the G light now goes out and stays out at idle.
Knowing that the output voltage is still quite low, I don't know how long this situation will last, or if it will sustain headlights *and* wipers, but it's working for now, which is a relief 
Meanwhile I have ordered an alternator kit and hope to do a planned swap rather than an emergency one.
Thanks all for the advice.
