If you haven’t already, explain your concerns to the insurer (politely) and ask if there’s anything else they’re willing to do.
I’d focus on it being a brand new bike and presumably insured for the full value, which has now been significantly reduced by the crash and repair.
This suggestion is coming from a customer service perspective rather than a legal one. Get them on your side and avoid accusations, e.g. saying the repair isn’t good enough.
If they’re still unwilling to help, maybe try citizen’s advice, or hope one of STW’s resident lawyers weighs in?