Dispute Resolution.

I did a job for a client and they added extra work but then refused to pay the whole bill after I delivered all the assets. I discussed the problem with Panos and he confirmed that I'd done all the work, and that my explanations were very clear, but the dispute resolution said that I had not completed the work and not offered to negotiate after sending over 100 messages over 4 weeks and then the client received an automated refund. I have tried to explain to Panos and Viki that they have made a mistake, all the assets were delivered, revisions made, and every item on the mutual agreement had been delivered. The client has only one feedback which is also bad whereas I have 50 positive evaluations and a 5* rating. I feel like PPH won't acknowledge their mistake and I'm looking for some ideas to escalate this situation. Has anyone else experienced this problem? The invoice is large so I'm considering small claims court and would be grateful for any information for solicitors.