Everything will get sorted in the end - it may be another week or two, but having sent in claims for hotel stays, car hire, online shopping in the past they do get credited eventually.
The only difference between bmi and an 'efficient' customer service organisation is that the 'efficient' organisation will divert its resources away from responding to comments or issues and send a letter of acknowledgement...
eg. 'thank you for taking the trouble to contact us, as we value your feedback greatly. We are experincing a high volume of customers contacting us at the moment and advise that a full response will be sent to you within the next 4-6 weeks. We thank you for your patience and your continued loyalty to our company.'
I think I'd rather hear nothing until the email saying 'the miles have been credited to your account.'
But that said I can understand your frustration!