Just FYI to this date I never had follow up on the first demand for payment when I misssd payment by accident. Don’t know if it’s the same firm chasing, but it wasn’t a UK based or swiss based company doing the chasing so I politely ignored them.

As you did actually pay - if you have prof of payment I would send it and tell them that’s it.

I doubt they have any chance to enforce though maybe others have recent experience.