Servicing of car

I purchased my VW polo 3 years ago and have only had it services at Tri-Sec Autoh aus in Trichardt. When I took my car for its service this year, I was phoned and told that my sump and sump plug was stripped and would cost me a bunch of money to repair. Because I did not want to do it through them, I was told that they could not service my vehicle until it had been replaced. When I questioned the lady who assisted me by saying that I only service my vehicle with them so how was it that my sump had been stripped if they were the only ones to have worked on it, she informed me that all the service techs had been replaced in the last 3 months and she could not give me an answer. As far as I am concerned, they are responsible for the costs of repairing this as they were the only ones to have worked on it and now claim to not be liable as the techs were replaced.