Appalling service

I gave notice to Discovery 1st May that I would be leaving. I was forwarded an email stating I owed an amount and this was paid. I have since been told I owe more money. Trying to obtain ANY KIND of help with this has been impossible. No one engages with me, I am just sent what look like automated emails. I am disgusted and WANT THIS RESOLVED!!!!!!!!!!!!!!!!!!!!!!!!!!! Who do I contact?