A few years back I had my car converted by a different company. Strangely enough though now I'm wiser I check out the registered directors of a company. It seems to me the ones that tend to build up a bad reputation set up new companies from time to time.
Checking these guys out, the listed directors are the same people I dealt with at the other company. That company did a bad job, failed on attempts to resolve. I ended up finding a far better converter locally who had quite a bit of remedial work to fix.
I sympathise with your situation, and it annoys me the people who cause it continue to trade without improving, it gives the good guys in the industry a bad name and causes the customer hassle and expense they don't need.
Keep at it, find someone how knows what they are doing and get it fixed. If you paid by credit card (which I doubt as they know) you can engage the CC company to assist as the contract is with them.
BTW where are you located, maybe the place I now use would be local enough for you?
Keep us posted and good luck.