I had this exact problem earlier in the year and it was the starter motor,get a mate and one of you sit in the car with the key turned and then get the other person to hit the starter with a hammer,sounds crazy but it worked on mine when i got stranded,so if this works you will def know its the starter at fault,my advice to you if it is the starter is get a bosch one,they are abit more but you get what you pay for.