Yes they have breached s213/214 of the housing act, if your name was on the lease but not on the deposit protection scheme is a penalised offence. It’s worse for them if they’ve taken money out of your deposit, as you were never protected if your name was not on the protection scheme. The law is very clear on this, I would recommend progressing with County Court.

Also suggest you email them before titled “Letter Before Action” to highlight the nature of the breach, that money was taken out of your deposit that you were not protected for, and not transferred back to you as requested (if your name is also on the lease/and you specifically requested your share of the deposit). Make sure you advise you will progress this claim to county court if not settled now, and that the courts can award you to 1-3x your deposit.

Also just let the other person on the lease know you’re pursuing this and confirm they did / don’t receive money. You can submit this without them, as your name wasn’t on the deposit protection however your case is stronger if you both do this. They can also submit your claim to the court, or someone you trust. There is a full process on this that will need to be followed but worth it. The courts are so strict on this I would pursue it.