as above, if its boiled dry its more than likely to be fucked, best to change it for the sake of £20 and knowing that new battery will give you (hopefully) years of reliable service .
as for the fault, check your regulator/rectifier unit for any signs of burning, shorting e.t.c, its a metal box mounted onto a metal plate with a plastic cover over the front connected by a four pin plug (1 red wire, 3 white wires), pay particular attention to the plug which connects it to the bike as this may have been damaged (such as in my case...).
now for the long winded bit...
The regulator/rectifier turns the high voltage (around 60v AC at 4000-5000 rpm) into a usable 13v DC output which can be sent to your battery to charge it, it does this by using a "diode pack" (think of it as a series of one way valves) to create a constant DC voltage, with the excess being sent to earth through a heatsink, if one of these diodes fails it will allow AC voltage output, to test this disconnect the red wire from the reg/rec and put an AC voltmeter from the red wire connection on the reg/rec to an earthing point, if it is displaying an AC voltage the unit has failed and requires replacing (mine was showing about 1.5v AC at idle, and hence mine is f*cked)