I telephoned the customer service number again and this time obtain the details for the 81213 provider and managed to speak to someone (unfortunately, not from the 81213 provider).
They have confirmed that I was subscribed to a company called VENISTA and their customer support number is 08706 091720. Unsurprisingly, this number goes straight to an answer phone!
They do have a website (http://www.venista.co.uk) which details an email address (firstname.lastname@example.org). I've added another message to their answer phone and sent them an email.
UPDATE: I received a call from Venista this morning at 11, unfortunately I missed it - but they left an answer message saying they'll try later.