Ex spent my money on herself

I split from my ex-wife january 2009,i have two children,we didnt get csa involved,my children visted every weekend and i gave my ex as much as i could because i was unemployed,in april 2009 i got a letter from the tax return saying that i had a return of 1000 pounds,but the money went into my ex-s account,she had spent the money on the children and herself,ok the children but herself i was pissed of about,i had a job last year 20 hours a week my rent is 500 pounds a month,the csa now say i owe 2300 pounds what should i do.