You can. I assume you were joint and severally liable for the debt.
Assuming the Bank does not have Judgement against your ex, you can issue proceedings.
You need to write a letter, set out your losses and request payment within 30 days or say you will go to Court. This is called a pre-action protocol letter of claim. You should make sure you send this signed delivery and keep a copy. You must give 30 days warning before going to Court.
If they do not pay you then you can issue proceedings in the County Court. You can either do this online at: www.moneyclaim.gov.uk or by completing form N1 https://www.gov.uk/government/publications/form-n1-claim-form-cpr-part-7 and take it to your local County Court.
The Court will then issue a claim which a copy will be sent to the Defendant who will have a limited time to defend it, if not you can enter Judgment and enforce.
If the claim is for £10,000 or less it will be a small claim so you will not need legal representation. Over this value, you would need representation for trial.
Can I clarify anything for you about this today, please?
If you need anything further I am available for a follow up at no extra cost.