British Gas old debt issues on credit file

I'm not sure where best to post this but hopefully someone can give me some advice.

Years ago I was using British gas for both gas and electricity in my previous house. I tried to move supplier to Scottish Hydro, British gas released my accounts but then notifies me my account was in debt so I quickly cancelled the change with hydro as it was still in cooking off period and asked them to return the accounts to British gas as I couldn't afford to pay of debt in full to British gas. This worked fine for Electeicity but there was an issue with the gas. This took soooo long to sort. Month and months went by and finally it got sorted. I lodged a formal complaint with British gas, someone got in touch to apologise. But we hadn't been able to pay anything to British gas during this time.

I can't remember all the details but eventually they did a meter reading and we paid off the account. I then split with my husband, I moved out and a few months later notified British gas that I no longer lived there, they changed the account name to my ex husband and told me all was fine and settled.

Today I tried to open a new current account, it was rejected. I was advised to check up my Experian as I have minimal credit, never missed payment etc. On my Credit file it states I have missed payments and outstanding debt since Feb 2015. It says I left the address and did not give them a new one and left debt behind.

Phone British gas, they are investigating it. Opening something called a CIAS??? I'm waiting on paperwork.

I am so lost on what is going on. Can someone help me!? I don't think I should owe anything but mostly I want this removed from my credit file!! They told me everything was fine when I changed the name on the account and no one asked for my new address or any payments outstanding.

Thanks