TNT UK trying to charge me twice

[tech-line-direct]

Dear All,

I have been getting overseas parcels from TNT UK, would normally get the VAT import duty invoices from them which I have all paid for. 

Recently they sent me an account statement stating that 1 invoice was not paid. I've checked the details and have found that I have actually paid for it. 

Since this email, I have received around 5 letters from them asking to pay the invoice. I have logged numbers of invoice queries with them providing the proof of payment, screenshots from my bank statements, bank slips, etc showing the transaction with the invoice number ( the same as they are asking me to pay for, for the same amount) and still nothing. Now they have passed my details over to the debt collectors Control Accounts. So I have sent the same over to them. Does anyone know any contacts of TNT UK where I can get in touch with them and speak to someone who can actually look into this? Because it looks like their customer service is very useless, they will either ignore all of my emails. 

In the last email, they said that now I need to speak with the Control account in regard to this matter because they don't deal with this anymore. I have contacted Control Accounts and asked if they have any updates. They said that they have passed the payment details to TNT but still have not had a reply from them. It's just seems No one is able to get through to them.