ER CONSULTING USELESS

They are the worst. I took my son into casualties last year July for concussion after rugby match. I received a invoice this year in May informing me that I owe them money. I paid full amount on 29th June but still they handed me over to attorneys after full amount was paid and proof was send to them. Now 11days after payment they are still telling me that it's not reflecting. Where can this company be reported as I've had enough of them now.