Finally got this sorted after 3 hours today!
Spoke to someone on web chat who confirmed that yes, the contract should have expired and the new one had been set up in error.
They passed me to the retention team who claimed this was a lie, the contract would run until December and I would be charged £190+ to cancel it. They offered to increase my line speed but not reduce the package, which would have tied me in for another 12 months.
I disputed that and said I'd be requesting a deadlock letter to refer the complaint to CEDR/CISAS. They accepted this was an option, said they'd do their best to help, and then passed me on the relations team who ignored my messages for an hour and then the chat ended.
So I tried again, started a new web chat, and was passed onto another colleague, who ignored the messages and the chat ended.
Around this time I tried to submit a formal complaint via the website, which results in a 606 error every time - convenient! "We have found an issue
Please give us a call and remain on this page as one of our friendly agents will ask you for your URL reference. This will help us understand what's gone wrong."
So I rang the number, and obviously no asked for a URL reference. I explained the issue to the agent who finally passed me onto Sophie in the UK who was incredibly helpful, apologised for the errors, reduced my bill by £11 a month and gave me a 2gb sim card to go with it.
😵