Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
What other steps have you taken to recover the funds?
Interesting technicalities involved.
Did you ever put funds in the account?
Technically since it is a joint account you can indeed withdraw the money.
However there is Civil Case law which suggests that the money is in fact held on Trust for your partner and that by withdrawing it you are taking money that does not belong to you.
However that is Civil case law and it is unlikely that the Police will look beyond the basic legal position so you have very little to fear from the Criminal Law side
Your ex could use Civil law to recover the money - but then you can counter claim for the finance figure.
I hope that this is of assistance - please ask if you need further details
You are most welcome I hope all goes well