Is my Ex-boyfriend entitled to half of the joint savings?

Before we separated I agreed with my boyfriend we would use some of our savings to pay off some bills, but we couldn't take it from the joint account as it doesn't mature until 2019. So I used the money from my personal savings and he agreed to pay it back. Since then we have separated. We didn't use all this money on the bills so he has had the rest of the money. I have tried to get him to sign a letter to send to the bank to close the account stating the amounts he is entiltled to, but he will not sign it. 

Is this worth me getting a solicitor to fight this, and what will it entail? Court or a letter? 

He is entitled to £2500, he wants £5000. 

I have bank statements that proove he has had the money from me.