Urgent, help needed: Car won't start

[URSIMPO]

After driving my car some 40 miles yesterday, I jumped in it to go to the shop. After jumping back in it to go home, I turn the ignition key and nothing. So I get a jump start of my mate who arrived to help. All was well, the car started.
So I put the battery on charge over night and today.
I connect the battery back up and the car turns over but quite slowly. After cranking for a few seconds, there is an almighty backfire.
So I attempt to jump start, the starter is cranking over normal, but then there is another backfire.
Yes I know a new battery is needed, but why won't car start. What have I done?
Any help/thoughts suggested, I'm going to Europe in it tomorrow night.

[s2_bo]

Sounds like a dead ish battery, its turning over slowly, but still injecting fuel hence the bangs.
Does the charged battery read 12v on the gauge when you turn they key in the ignition?
Try a new battery and see if that solves it.

[URSIMPO]

New battery fitted. Works like a dream. Phew