Device repairs

I am very disappointed in the way in which my query was handled. I booked in my cellphone for repairs and when it came back, I reported that the lock/power button was faulty. The technicians who assessed the phone for a second repair came back to say the phone had been damaged after the first repair and this damage affected the lock/power button - the "damage" they are referring is a scratch on the side of the phone which was not there when I booked in the device, I am now being told this will be a new claim in which I will be liable for excess. How do I know the device was not damaged by them in order to avoid taking responsibility for a faulty repair? I am very frustrated by this company and it is not the first time I have had difficulty with a claim.