We had this problem when we moved home about 6 years ago. There was a heatwave and all the windows were locked but we had no keys. We had to replace all the window locks. By putting a larger amount of force but not too much on the handle you will find it the pin will snap allowing you to open the window. Window locks can be purchased separately. I assume your brand of windows are similar.
John Carr as a independent brand ceased to exist years ago and is now owned by Jeld-Wen. See the details on the link below.
I would email (or phone) the 'Windows' group customer service directly from this link - they would be the best people to ask. http://www.jeld-wen.co.uk/?cmd=main.contactus