The thing with battery lights is they are just useless. half the time even when the regulator pack fails the light still wont be on, the light can only be on if the output from the alternator is fully earthed.

Your problem sounds a lot like a car i had years ago, the light never came on and the alternator was charging, but the brushes were seriously worn and it wasnt charging at a high enough amperage, sat in traffic etc once the cooling fan came on a couple of times the car would cut out because the alternator couldnt supply the current needed.

you'll probably find your alternator is bosch and usually they are very easy to change the brush/diode pack on, costing you probably £15 - everyone seems to run out and buy a new alternator for £90+ in these circumstances, and is more often than not a waste of money.