Struggling with incorrect invoicing

Really struggling to get a corrected invoice from Pathcare. I had a shortfall on their account and paid over the amount to them. Then submitted the account to my Gap cover, to be told that they cannot reimburse as Pathcare has billed me for mutually exclusive tests, and they need a corrected invoice from Pathcare. Emailed them on 19/8 - no response Followed up on 22/8 and was told by Lauren Harris from account that it has been escalated. Followed up again today 26/8 and received no response. At this rate, my Gap cover will close their file due to no-response from me and I will have to accept losing that money. What confuses me though, is that Pathcare is billing for tests that apparently cannot possibly be done together.