It could be a combination of both - where abouts in your back are you getting the pains - in your lower back or higher up? Are you getting any shooting pains or pin & needles or numbness in the backs of legs or buttocks?
It's the lower back from the back of the ribcage down. The pain is like a dull ache down either side.
I'm not having any of the other pains you suggest, it's only the lower back. A work colleague of mine who did sports science at uni suggested that it may be down to poor running shoes, something else I may have to look into.
Thanks for the stretching links, but I can't read them as I'm not a runners world subscriber!
Good that means you are unlikely to have sciatica. Sometimes the last two vertebrae in your lower back can get locked up with the impact of running - this is why your shoes are really important. Did you buy your shoes from a proper running shop? If you have a desk job as well you will also need to look at your work station and make sure it is set up correctly as well.
If you PM your email address I can forward those back exercises to you as a PDF.
By proper sports shop I hope you mean a running shop that does gait analysis - make sure they do gait analysis. A lot of running shoe companies are currently updating their lines so you may be able to pick up something in a sale.