What about the debt collection agency demanding that I pay their administation fee?

Any help would be appreciated.... My partner had a small invoice of approx. £180 sent to her by Vets Now Emergency Care and that she agreed to voluntarily pay in small monthly instalments of £25, something they accepted but didn't agree to as they just kept on telling her that the invoice must be paid in full. She had paid £75 over 3 months (obviously their 90 day period) and then she received a letter from a debt collection agency informing her that they were now pursuing the debt on behalf of Vets Now. The debt was now £180 Once she received the letter I asked for a full statement from the debt collection agency and then on receipt I paid the original debt only. However my partner is now being perused for the outstanding balance equating to charges of £75. Ive talked to them on the phone explaining my belief that that they were only acting on behalf of Vets Now so were not entitled to add charges however they refuse to discuss the matter and just keep referring to their account not being closed. My partner has not received any further letters as of yet... however once she does we will be able to respond in writing formally. How should I approach this ? We can pay the outstanding balance but I don't agree with the working practices of these companies and im not paying anything that they are not entitled to. Also....... any chance to give Vets Now Emergency Care a bit of bad press is certainly welcoming......