Seat Toledo 1.9 TD Class 1993

On Thursday, July 10, 2014, I took my Toledo to a vehicle inspection station (ITV) I was unfamiliar with, as it was cheaper. Among other things I had to put up with was a young man slashing the engine while it was still relatively warm. The sudden acceleration he gave made the engine roar like nothing I'd ever heard, giving me quite a fright.
Since then, the engine runs overheating. Before, the temperature gauge would stay at 1/3; now it's at 1/2. Please let me know if this is the cause. Thank you very much.