Dion wired Woodmead Samsung repair centre

On Friday, 22 December, I took my fully functional Samsung Note 5 smartphone to the Dion Wired Samsung Repair Centre in Woodmead, to get my battery and back cover replaced. I was advised that the repair would take a maximum of 60 minutes. When I returned to the store to collect the handset. the technician advised that the phone was not switching on, and they were investigating why this had happened. I have followed up on numerous occasions between Friday afternoon and this morning (Tuesday, 26 Dec). only to be told this morning that they are unable to repair the handset under warranty. I have now been advised that there would be additional charges to repair the handset in order for it to function again, over and above the initial quote for the battery and back cover. I do not feel that this is fair, considering the fact that the handset was taken in for a specific reason, and I should not be held liable for the additional charges incurred, due to negligence on their part.