Fix water leak

I bought a house in Melhus (close to Trondheim). There was a water leakage in the house last year, when already no one was living there anymore. They have simply shut off the water, without fixing the leak. i will get the keys on 28 July, and of course i would like to have the leak fixed as soon as possible after that. i am thus looking for a plumber that can come to Melhus sometime in early August. The leak has to be found and fixed. They told me that most likely it was the sink in the toilet, which doesn´t sound like a very complicated job to me, but of course i am not 100% sure about that.