Haven't oiled my locks lately (doh!) so some of them became hard to open. Took a window mechanism apart, turned out to be broken teeth on Saracen 9202158 Roller latch gearbox (61mm) Ordered a pair from local UPVC Installer. Cost £40 a pair as they had to be ordered. have since found them on line for....£8.50!! (was in a bit of a rush)They are easy to replace in less than half an hour. Check that the shootbolts aren't worn as they'll need replacing at around £5 each. Use silcone grease not 3 in 1 or any other light oil. I hope this helps.
Jerry
January 2009