Probably just a shagged battery then, i had similar problems a while back with having a flat battery every few weeks, had to keep getting a jump start, and i thought it was loose connections, not charging, etc.
Most likely a dodgy cell in the battery so it's just not man enough for the job, and you're right, it will die a death in winter.
Bought a new one and has started first time without issue ever since i've had it, but as i said before, after learning how much power the lights drain the battery i've got into the habit of switching them off when the bike cranks over to take away that added effort.
If you do order a battery be careful that you check the physical measurements, as i found that the codes can very slightly differ resulting in a different size battery, and there's not much room to play with.
I chose this one because it was 12ah, whereas a lot were only 10ah.
http://www.ebay.co.uk/itm/VARTA-YT12B-BS-12v-12Ah-AGM-Motorcycle-Battery-/311057282347?_trksid=p2141725.m3641.l6368