if you get it open take note of any brand name and stock numbers that are printed on it,then order part that has broke,you can get most parts from north west hardware website,it sounds like your lock is in 3 parts and the gear box has broke.If you feel confident enough you can remove all screws and take lock out yourself and try to repair but make sure you know what you are doing first as you could be left with a window that wont lock.I run a company that does this type of thing and if its what i think it is the price would be around £50 to fix,so if you could ring a few double glazing repair companys in your area and get a few prices first.if you do yourself and get stuck you can use my websites contact page to e-mail me with prob.hope this helps
shaun www.sdwupvcrepair.co.uk, February 2008