Landlord withholding deposit needlessly and being petty

I moved out and the landlord Kevin Scally completed the final inspection. He came into my new place immediately after which is next door arms length away for a chat and my new landlord was there coincidentally. I know it only takes 30 seconds or so to activate the deposit return on SDS so the night after he left, I activated it since he hadn't. 4 days later I hear him go into his place so I asked him what's happening with the deposit. He got all frustrated and pointed out a stain on the carpet (which were done when I moved in anyway and I asked several times if I could buy new ones myself). I know he knows the stain was already there. It would come under fair wear anyway as it is at the door entrance and it's a cream carpet and there was no rug to protect the carpet until I put my own one down. We argued and I surmised that he is being petty and a time waster. He said to clean all the carpets it would cost around £70, so I assume the stain can be done cheaper. My deposit is £750 which I obviously need back to make up for the costs of paying for my new place and furniture as it was unfinished. He has came up with some more lies today saying he can't release a part deposit which I know is BS. Guess I'm going to just have to wait it out. Also the realtor has shown at least 2 potential buyers the property today so he's clearly happy with the condition of the place.