noggs

Hi, First a bit of background - I've got a 2001 Fazer 600 which has been running well until recently. It was out in a torrential downpour and the rain shorted the alarm which drained the battery. I've since disconnected the alarm and it's been ok but the battery took some permanent damage. The bike had sat there for a few weeks and I tried to start it last week and the battery was flat. I jump started it from a car but then after sitting outside work all day it needed jumping to get it started again. Guessing it was the battery I replaced it this weekend. The new battery is now fully charged but it still won't start the bike. I can hear a rapid clicking coming from the relay, and the starter turned over slowly the first time I connected the battery but then nothing after that. Just the clicking. I connected a meter across the battery. 13.6V disconnected/off, drops to about 13V with lights on, and drops to slightly under that when I attempt to start it. Based on this it looks like the battery should have enough power to start the bike. (The old battery, even fully charged, drops to 5V when starter button is pressed). The battery is not the same model as the official battery it used to have - but I'd have thought that it would have started the bike. I can still jump start the bike from a car - just looks like either the battery is not upto the job OR the bike is drawing too much current to start. Does anyone have any advice for starting to diagnose this problem? I'm thinking maybe starter motor, or ignition system, but I don't want to replace parts needlessly. Thanks for reading!