The problem now is that if 50C give you a replacement bike and you post that on here, everybody will want one, and that will put them out of business. The same if they give you your money back. many dealers will make "special" arrangements when customers need their problems sorted out, but they usually want you to keep quiet about what they did for you.
Refusal to send emails doesn't make any sense. They have to communicate somehow, and no matter what medium was used, you could still photograph or record it and post it here.
If they refuse to communicate completely, you only have one way forward, and that's a final demand followed by a county court action. County court cost £60 last time I used it. The £60 is added to your claim, so if found in your favour, you get it back. Online county court is dead easy to use. You only have to write down who owes you what and why. The CAB will help you with that. Then pay the £60. You don't need any solicitors or anything like that.