if it is upvc windows a key cutting place should have them I think, we went to get one cut once for work and we were told they were standard keys and just passed us one from the shelf! Do you have other windows with locks in the house? Could you try one of those?
Yes, all other windows in the house have locks & keys, and believe it or not it is a different key for every room in the house, none fit the living room! Guessing that the previous owners had the windows done at different times. They are UPVC windows.
Yeah, we moved into our place last year and don't have a key for the windows, but we are a first floor flat so I didn't bother doing anything. They can't be opened from the outside either, but may have to re-think when ds's are old enough and big enough to get to them