As per the other thread tbh mate. He’s not allowed to make a profit from the works to replace the locks, so even if you are liable you want to see receipts of the work done. If he hasn’t done the work then he needs to submit multiple quotes to the deposit holding scheme.
In truth its not up him or you or us, who is liable its down to the people who maintain the scheme. If he has not used a scheme then regardless of who is liable, he has broken the law. Don’t mess about with him any more tbh ask him for information to allow you to progress.
If he cant do so write to him giving him an acceptable time to provide you with the information to resolve this and if he cant, report him.