The battery runs flat quickly after charging. The bike eventually stops - after a few seconds i can push it to start an drive on, if i disconnect the headlights... Where do i start searching for errors? I suppose this goes beyond the battery...

You are riight that it is not just the battery.

You need to check the regulator and see if it is regulating the current form the generator. Also, I would have a good visual look at the wiring form the generator all the way through to the regulator.

These faults are common, but easily fixed.

stick a search in for "reg rec" and you will get your answers.

basically..new battery, new reg rec, possible generator...

thanx

get a multimeter,charge the battery up,reconnect and check to see if its getting charged.
if not then more than likely the reg rec......the charge rate should be between 13.5v-15.5v@5000rpm.
if the battery is getting a slight charge but not great then suspect the alternator to be faulty.....
but do this check first.
:D