Together Energy - changed MTC and can't move supplier

So this issue has been dragging on for nearly a year. I moved to Together Energy when they first started up and wished I hadn't. Had issue after issue with incorreclty recorded readings, no bills or updates.
I decided enough was enough and I'd move back to Scottish Power because at least I knew I could see what was going on with them. The transfer of the electricity didn't go through because I was told Together Energy had changed the MTC code. I've been told as my current energy company they are the only ones that can change it. They keep saying they will get it changed and nothing has happened. I've even tried to raise a complaint via email and over the phone and got nowhere.
Any advice on what I can do?