I've never had this problem, but I have some advice. There are three possiblities that I can see:

1. Someone you know or who has physical access to your computer is to blame
2. You have spyware such a keystroke logger on your computer. If you don't already have anti virus, firewall and antispyware software, GET IT.
3. Someone with access to the bookmaker's site (employee, other), got your details.

IP is everything here. You can find the geographic location of the IP by typing it in here.

If the IP is close you have your answer, if not it's likely 2 or 3.

If you don't have it already, get microsoft antispyware (free), run a search on your system. Also get lavasoft adaware and spybot search and destroy. You may just find a keystroke logger. If so I'm not sure where you stand with regards to getting your money back. Also for the future get a firewall, you can enter privacy data (such as your password/banking details) so you will be warned when it is being transmitted over the internet.

If you're clean on these fronts it's probably their fault. Explain what software you run, your history etc and they should credit your account.

If the IP is local, you can contact the ISP who owns the IP and explain your situation. If they're not forthcoming, your only recourse is to contact the police. Faxing a copy of the police report may help get your account re credited - in most cases I've heard of they'll refund honest victims.