As per my post in the 500 thread, it's the battery.
Cold weather is pushing it over the edge. Order one online and fit it yourself.
Once, I would have quickly agreed with you, however, I'd like to give the benefit of my recent experience of buying a battery online.
At the beginning of 2013, the factory fit battery in my Saab got killed by the cold weather so I ordered a new one from an online supplier. It arrived next day. I installed it and the car to all intents and purposes behaved normally, good starting etc. However over the last couple of months, the car had been suffering from laboured starting and then last week, first snap of very cold weather and the car was dead. Embarrasingly, I had to get a jump start off the wife's i10!
So next day, I went to my local motor factors and bought a brand new Yuasa battery, charged it fully overnight on my trickle charger and popped it in, no probs, everything is rosy in the Saab garden. I know my alternator is working ok because my Scangauge is showing a good voltage on the battery and it indicates it is charging the battery as it should.
What's the point behind this story? Well before I installed the new Yuasa battery, I managed to decode the manufacture date, it was made in July of this year, quite literally just 4 months old from the factory. The Varta battery that I had bought online, well prior to returning the battery to the motor factors to get my deposit back, after almost an hour of research on't 't'internet, I discovered that it was made in March 2010! Yup, the battery I bought online, was almost 3 years old by the time I got it. I was quite literally fuming. It lasted just short of two years, diabolical for a battery that came with a 5 year manufacture warranty. I can't even pursue a claim on the battery now because I've handed it over for recycling.
So if anyone does decide to go down the line of buying a car battery online, this is just a forewarning. At least if you go to a local motor factors, if it goes belly up, you can take it back easier than if you bought it online, plus, you can at least have some way of checking the date code on the battery. I'll never make the same mistake again.