cancelled bank card and checked no DD set up.
All they’ve done is sent you an email. If they were a legitimate company and you’re not a customer then they wouldn’t have taken any money because they don’t have your payment details. A scammer who did have your payment details they wouldn’t take the money and then politely email you, under the guise of a broadband provider, to tell you.
The scam isn’t taking £836. The scam is you clicking on any link in the email thinking they are a legitimate company to deal with and giving them any information. Hopefully you’ve not done that.