2005 Peugeot 206 Locking Problems

Hello, , I hope you can help me with this.

I bought a 206, 2005, 1.4. Cars running great, with one problem which is beginning to annoy me. When I bought the car I knew the key fob wouldn't open/lock the doors. I was hoping it'd be just a new battery, however not the case.

I can lock the car from the passenger side, however from the drivers side, the key will go into the lock, but it doesn't turn at all.

Do you know what's wrong, what I need to do to repair, or if I should go to a mechanic how much it'd cost?

Many thanks.