Phone will not charge up

Common problem, I've had countless charging problems with Sony-Ericsson.
Is this USB or Mains charger? Mains charger seems to be more effective. I had a problem with my K800i where it ran-out of battery completely, then when I charged it, the screen would stay white. I would have to keep it there for an hour or so, then take it off, turn it on, then charge it again and it would charge fine.
Another problem I had was with a K750i - the phone would state it was charging, but when you took it off charge, the battery would be low. I think the battery was just old in that one. A new one may have sorted it. I took it to my nearest operator store and they sent it off for repair. It took around 2 weeks and it still wasn't fixed, so I got a W810i as a kind of 'Sorry your phone sucks' free replacement.

So, how old is the phone? the battery may need replacing with new, they're around £10.
You may be able to send it off for repair.
Try a different charger from a friend or local store - just ask!
Try the method I explained for my K800i.