Hi Doug and welcome to ECF. I'm terribly sorry for your terrifying ordeal and I'm glad you (and your backpack) are OK.
I always store my spare batteries in a special pouch. I highly recommended you get something similar and never, ever just toss a charged battery into a soft pocket of your backpack--I suspect that a little fabric will not protect your battery from coming in contact with other objects you carry in your bag:
Battery Cases : Nylon Battery Case w/ Carabiner
And a bit more info on basic battery safety:
http://www.e-cigarette-forum.com/forum/ecf-library/129569-rechargeable-batteries.html#post2029353
Thanks for the advice Katya, I'll certainly see if I can get hold of that kind of case in the UK. It was terrifying! my bag is toast and I'm embarrassed that I nearly evacuated the entire office... but very glad no one got hurt