agree with the others. it is a mistake on their part but it dosent mean you are entitled to the money. If you accidently put the wrong account number down when paying in 6000 pounds and it ended up in my account, would you really expect me to keep it? Like them, you would have made an honest mistake and you would expect it to be recitifed. The only possible thing you can do is ask for more time to pay, if it's going to put you in severe financial hardship...
You dont say how long this error was accumulated ove, but didnt you realise?