Dad bought car for me but husband's name was down as registered keeper, how do I get car back?

Does your dad have any proof of purchase for the car?

Yes he paid for it with his bank card and has his bank statement but the actual bill of sale is in the car pack that my ex still has in his possession.

Ask your dad to give him a friendly call first asking him to return the car. If he doesn't, then you can take legal actions against him and use your dad's bank statement showing the car purchase transaction as proof.