MFI are a big company so should have a complaints procedure.
Try speaking to them on the phone(noting down all times and names), but the best thing to do is write to them, then they can read exactly what has happened.
Include any photocopies of receipt and warrenty for evidence, then hopefully wait for a response.
Always include that you are a good customer to MFI and hope this can be resolved as you have high hopes in the company. They'll want to keep you sweet, so hopefully something can be done.
And you still haven't lost your consumer rights, just because it is ex-display