A couple weeks after you sent the first letter, you could follow up with a less harsh (toward the company) letter, phone call or e-mail. If you don't get a response from whomever you wrote to, try going up the company hierarchy. Probably the best advice is to think about what the most effective way would be for a cyclist to complain about one of your company's drivers. Then do that to the bus company.

I think you should insist on a response.