Hi, same thing happened to a window in my mothers house. The external locking mechanism snapped where the handle meets the it. I contacted a local "windows install" guy & got a second hand one from him for about €8. It's just a few screws keeping in the locking panel in the window, but access is from the outside of the window. It happened on a bedroom window, so I was able to access the window ok as it open sideways out, does your bathroom window (upstairs I assume?) open from bottom outwards or from the side.
The one I needed to get was an espannolette lock. Reference here... There should be numbers for the size of the lock itself... Handy reference is: [broken link removed]
As I said, there is very little to do to fix this, assuming you can access the lock to take it off... also, your handle needs to come out. Just take note of how it fits together as you are taking it off. Some windows repairers will charge hefty prices for this, call out charge, might insist on replacing handle etc...