Sending legal consumer notice to a hospital

My mother recently underwent a surgery (called Deep-brain-stimulation) costing 11 lakhs out of which 8 lakhs can be claimed by group Medical insurance. Since the claim is post-discharge, Insurance firm is asking for detailed breakup bill. Hospital has given me a packaged bill in the begining and after lot of request an approximate break-up bill. The main issue is there is an item "drugs & disposable and devices" in the approximate break-up bill. The device costs Rs. ~6 lakhs (which covers most of the total amount) and the Insurance company is asking for the original Invoice. Hospital is not providing it citing some weird reasons. Hospital is also telling me that the packaged bill is accepted everywhere and it is the problem with the Insurance company.

Finally I'm tired of running between Hospital and Insurance cmpny and planning to send a legal notice to the Hospital. I believe I'm entitled to get the above mentioned Invoice as part of the bill.

Pls suggest me the proper procedure here? If the hospital replies for the legal notice (like a valid legal reply telling to accept the bill already given) then should Insurance cmpny comply with it ?