I had something similar happen with a 'cottage industry' supplier on the other side of the world, who was always too busy to refund me for my trade-in parts. After several months I wrote:
I am still waiting for my funds. If you can tell me what the problem is at your end, my partner may be able to help. She managed international transactions for one of the big four Australian banks.
Alternatively, my brother lives in England and is a solicitor, so could act as an intermediary
I got my refund overnight (or I will have, after allowing for bank clearance times).