Filligree

Hi, I recently bought a 1995 import SRX400 (at 33bhp, it's perfect for my pesky restricted licence, and my lack of height), and have been using it to commute happily to work, as well as taking it for a trip to the Isle of Man in June. A couple of weeks ago I went to the bike after a heavy rain storm to head home after work (it sits outside at work, but inside the garage at home), only to find that for no apparent reason the battery was flat - all the idiot lights, but not enough to start the engine. So, swapped the battery for my other half's CBR600 battery (which is identical), it fired up, and off we went. This morning, I got it out of the garage, turned on the ignition, pressed the start button, and got not so much as the start motor noise before the entire bike went dark. So, turned the ignition off, then on again a couple of times without any response, tried again and got power, pressed the start button and the same thing happened again. I've hunted round the 'net for similar problems, but this seems to be a new one. Can anyone help me shed any light on this please? Filligree